> me for pointing out bugs in his software that he refuses to acknowledge
Sometimes this is a perspective thing. It's not always a bug just because some user reports it as a bug in their opinion. Don't know the story behind that anecdote, but perhaps you didn't understand the codebase like you thought you did, or what the expected behavior should of been?
In any regard, if the project maintainer does not consider it a bug and won't accept a PR, then that is their prerogative. You are [usually] free to fork the codebase and fix it yourself if you had a PR that wasn't accepted. With closed source, that isn't even a remote possibility.