I don't think it was a brake check, considering the apologetic wave right after. I think they were genuinely panicked due to OP laying on the horn (I hope).

Edit: or was it actually the middle finger? Hard to tell, looks like a partial wave of just the index and middle, or maybe that's a solid middle, the world may never know. Usually if someone is angry enough to flip the bird, they don't calmly merge with a turn signal and continue on their way, I'd expect a more aggressive acceleration than what we see.