They go into more nuance in the comment they linked within the comment I shared which addresses their experience with some of the things you mention, where publishers will change/add things, and that new material changes the public domain status, but others will change minor details and and try to call it a new protected work.

I’ve seen many guitar tab sights get copyright noticed out of existence, but now playing piano and learning about IMSLP, they seem to be very above board and respectful of the law, so it’s interesting to hear of the challenges they face even in trying to comply with established rules.

Things like what IMSLP provide are at least as much educational and historical materials as they are entertainment, and I’m glad they’re trying to legally preserve it all. I’ll have to look more into their difficulties, it was very interesting reading these 2 posts and their content is very much of interest to me.