Does this sentence make actual sense? (That "than someone else" part looks unnatural)

That totally makes sense. Although “someone else” would generally be a specific person/thing “Steve was closer to Mary than Jim”

Yea it makes actual sense. I can understand it

Yeah it makes sense, but it does feel a bit weird. I would think it would be "Steve was closer to Mary than anyone else." Or maybe, "Steve was closer to Mary than Bob". It feels weird to leave the 'someone' unnamed in this sentence.

A lot of people are mentioning that sentence makes sense, but I don't think that's what OP is asking. Yes, it makes sense, but it's a bit of a weird sentence. It definitely has a function as is, but I think you would typically use something more specific or change "someone" to "anyone". It really seems like the app wants "someone else" to be a placeholder.

Yes it makes sense, especially if [someone else] is like a placeholder for another name. Personally, I'd use "than anyone else" suggesting you're asking the closest person instead.

It's an annoying sentence really, because grammatically there's no problem with it and it does have an intelligible meaning, but I just can't really think of a likely occasion when it would be useful. In an English learning context - in my own tiny, small, singular opinion, anyway - it's a missed opportunity not to embed a word you're trying to teach in a sentence the person might be likely to see, since that helps them understand relationships between words more effectively. But I suppose I could see the argument for advanced speakers being exposed to grammatically-correct sentences of dubious actual meaning as well. Actually now that I'm this far into the response I suppose I could invent a scenario in which it would make sense, but it would be such a tortured, convoluted, labyrinthine setup that it would no doubt be painful to read, so I'll spare you. Anyway, to conclude, you're right, it's a weird sentence with no obvious utility.

It is in fact grammatical; however, the flow is very strange. I don’t think anybody would ever phrase it this way because it’s set up as if he’s closer to Mary than he is to someone else in particular. It would sound more normal if it was “closer to mary than he is to Barbara” or “closer to Mary than he is to any of her friends”

I think “than anyone else” would be more appropriate 

“Someone else” should be “anyone else.” It means there was no one closer to Mary, than Steve. “Someone else” specifically introduces a new unknown person. As written, it means Steve is closer to Mary than… some random person whose name we don’t know. That may be grammatical but it can’t possibly be what they meant to say.

It’s a weird sentence. It should be “than anyone else” as a standalone sentence like this. I can’t think of a situation where sentence as written would ever be used, since it has an awkward, unnatural construction. I’d almost go so far as to say it’s downright wrong, semantically. This test seems like it was made by a non-native speaker.

It makes sense. It's ambiguous but it makes sense. Does it mean he's closer to Mary than he is to someone else or that he's closer to Mary than someone else is to her? Either way it's a perfectly natural sentence.

It’s grammatically correct, but it is a bit odd 🙂I’m not sure when you would actually use this in a sentence. I think this one is intentionally vague where we don’t care about the “someone else”. You could easily replace “someone else” with “anyone else” and have it mean the same thing. Essentially the sentence is saying that in the unspecified crowd, no one is standing closer to Mary than Steve is. If you were to modify the sentence to add “to” then the meaning would change: “Steve is standing closer to Mary than TO someone else”. In this case the meaning is different. This would mean that in the unspecified crowd, the nearest person to Steve is Mary.

It's right it's just like there's not likely to be a situation where you would say this. This implies you know the specific other person but just aren't naming them. In general you would name them or speak generally about the whole population and say "than anyone else". It's like saying "Steve is closer to Mary than the planet Jupiter". Well sure yeah, but why would you say that? What meaning or use does this have

May I know what app is this? :)

Whoever wrote that app is not an English speaker. No one would ever say this.

Yeah it's grammatically correct but sounds weird. It would sound perfectly natural if it was "than anyone else", but I don't think that was the intended meaning. It looks like "someone else" refers to a specific person, and if that person replaced the "someone else" it would sound just fine.

Definitely odd to say that way. I'd say that Stever is closer to Mary than any other. Or perhaps closer to Mary than Jack. But if "someone else" has to be preserved for some reason then I'd say that Steve is closest to Mary than someone else.

“Someone else“ could be replaced with anything. It could be replaced with another person; it could be replaced with a physical object. In the case of another person, it could be referring to physical proximity or emotional connection. If it were replaced by a physical object, it would only be referring to physical proximity. You would only say this sentence without a specific thing in mind if you were saying that he and Mary have a stronger emotional connection than anyone else he knows. And in that case you would say “anyone else,“ not “someone.” Saying “someone” is weird.

In natural language you'd probably name the person specifically, as in "Steve was closer to Mary than John"

Click on the image to see the complete screenshot.

Yes, but the meaning is ambiguous, because it's not clear if he was closer to her than he was to another person OR if there's another person close to her, but he's closer.

Yes. in the comparative form it is -er..... than. Jim is taller than Bill. The sun is brighter than the moon. The desk is closer to me than the table. Superlative from takes -est: The table is the closest surface to me. I am the tallest in my class. The sun is the brightest star. etc etc.

“than anyone else” would make more sense, but I do thing your example is technically incorrect.