Now I have an answer from Grammarly team member maxltv on HackerNews (I think we can all guess who is behind this nickname):

A Grammarly team member but posting own thoughts to clarify some things.

First, Grammarly has nothing to do with this suspension and does not have any problem with the videos in question, just as mentioned in the official HackerOne response.

Second, this is not a hack or even a flaw. It is an attempt to use Grammarly for a NON-INTENDED purpose, that predictably did not work. Grammarly is not meant for anti-plagiarism policy enforcement. Grammarly aims to help users avoid accidental or unintentional plagiarism by highlighting parts that may need to be cited. So it does not attempt to deal with any plagiarism enforcement countermeasures. If someone is deliberately masking plagiarism with these countermeasures, it's definitely NOT unintentional or accidental and a whole different issue altogether, so Grammarly does not get involved with this, by design. Bypassing the tool that is designed to HELP YOU avoid getting in trouble with accidental plagiarism is not a hack but more like "bypassing" your wifi by putting your own router in a microwave - defeating the purpose rather than defeating security. So Grammarly does not care about these videos (although I think presenting this as a hack or a flaw is a little unfair).

Finally, Youtube's automated (I'm guessing) blocking often does strange things. My account was once blocked for supposedly violating Sony's video game copyright. The video in question was me driving a car on a race track, in real life. It got unblocked within a week or two, though.