More realistically, users are going to say "Hey Company X, this feature is broken." They won't know or care about libY. I would have replied with "There's no warranty whatsoever. Please submit a bug report and we will prioritize it accordingly. We do accept pull requests."
The bug might have low impact in most cases but doesn't work with how Company X is using libY, so it might not get fixed for a while. If this is hurting them, they can fix it themselves and submit a PR. Or they can work with them to prioritize their bug, which puts them on the other foot. If it's a huge problem that affects half the web, then Mr. Z will be working on it anyway.
If I were Mr. Z, I would know the problems Company X will have replacing libY with libW, and wish them the best of luck if they bring it up. No one's paying me, if they want to use something else, good riddance. Especially if they are threatening me. But I get it, people are different.
The bug might have low impact in most cases but doesn't work with how Company X is using libY, so it might not get fixed for a while. If this is hurting them, they can fix it themselves and submit a PR. Or they can work with them to prioritize their bug, which puts them on the other foot. If it's a huge problem that affects half the web, then Mr. Z will be working on it anyway.
If I were Mr. Z, I would know the problems Company X will have replacing libY with libW, and wish them the best of luck if they bring it up. No one's paying me, if they want to use something else, good riddance. Especially if they are threatening me. But I get it, people are different.