The guy you're arguing with made the mistake of switching to metaphors. The result is several back and forth posts, and neither mutual understanding nor useful discussion.

To avoid metaphors: There's an implicit assumption that when you install a piece of software, it will not fuck up your data/system without your consent. No, installing it is not consent, neither is using it. Neovim does exactly that. No amount of food analogies or license quotes will change that fact.

Neovim authors should've just used a separate file, just like they're happy to use separate format and files for the configuration. Instead, they've decided to be dickheads, and interfere with vim's file, even after the issue being pointed out to them.

My guess is that as a fork of Vim they’re intending to replace Vim and therefore see no reason to try and coexist with it. They would probably say something like “why are you trying to use Neovim and vim at the same time?”

Whether or not that’s a good design is definitely discussion-worthy.

I’m not sure I agree about the concept that programs promise not to fuck up your data without your consent. Programs can’t really know what you are consenting to. The authors can’t assume you’ve read the manual or understand what the program was designed to do.

I think there’s a decent argument to be made that persistent undo files are more akin to metadata, similar to Microsoft Word recovery files or search indexes, but I haven’t made that argument because the author’s frustration on the issue seems logical and valid. It seems like it’s still not meant to be a data storage or backup mechanism: you wouldn’t share it in an email or check it into source control.