What does "strike" mean here?

I think this usage is similar to "strike a pose." It's saying that he's showing off a defiant tone. 

I would say [definition 20.](https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.merriam-webster.com/dictionary/strike&ved=2ahUKEwiBrbnPxO-KAxVUFFkFHSzCO70QFnoECCgQAQ&usg=AOvVaw3rUGJYc9QuGdtO8kr4d1hB) Strike is a word with a lot of definitions.

This is not the technically correct answer, I reckon. But in case no else mentions it, I would add that to me this sounds almost metaphorical, like striking a chord: a defiant chord.

He is speaking strongly. It's like "striking a pose". He is adopting (choosing) to use a certain tone of voice, to express his opposition. Like Gandalf saying "You Shall Not Pass".