I once saw a video of some experienced fighters practising some technique on an instructor who had a training knife they don't know about.

With all the excitement and adrenaline, not only did every single student get 'stabbed' - many of them didn't notice until the exercise was over and the instructor pointed out the 'stab wounds'.

I'm not sure there's any amount of BJJ that would make me confident to grapple a guy who could have a concealed knife.

Yeah, I think that's exactly right. And it's hard to reach for a weapon when you can't move your arms.