It’s about as rock solid as a case can get. That’s why I always emphasize the importance of the DMCA and the danger of using an entire copyrighted work. Most of the time, a DMCA takedown is enough, but if someone wants to go scorched earth, they can skip right over that and head straight to a lawsuit.
I work in entertainment law, and while copyright is its own lane, there’s plenty of overlap. I bring up YouTube reactors because I see this happen all the time with music videos. And I’ve also seen artists drag reactors into court those reactors lost every single time. The deciding factor in all those cases was they played the full work. Even if they added commentary and tried to make it transformative, using the entire video is a massive strike against fair use.
Now, these cases are rare, because lawsuits are incredibly expensive and most of the people being sued couldn’t realistically pay damages anyway. But if a band has enough time and money, they’ll win. Most of the time they don’t pursue it. Because sometimes they have deals with the reactor through the record label, or it can be free publicity, yada yada. Or sometimes you just end up, losing more in fees, than what you would recover. (that is very circumstantial though)
And in this particular situation, it’s even worse, the reactor admitted they were acting as a market substitute. That adds malicious intent, which means they’re looking at punitive damages stacked on top of everything else.
So if you want to react to something, I can’t stress this enough, use snippets, use anything that could be considered contextually ok, but for the love of God consult a lawyer if you need to, and never, ever, ever, under any circumstances use the entire work.