Now, some users will perform due diligence and maybe find the NixOS issue referenced in OP and look elsewhere, but many will find the project and immediately file an issue there in the upstream, leading to the author's main concern of "I don't have time to support NixOS issues".
I was thinking that people will do the same or else how will they know if the github repository returned from search results is truly the one installed?
So for example if this will be the case with a package I use I will report the bug to the fork because the package manager showed me their link.
I will not search for name-like projects or the original one and report there if a fork was distributed to me.
This is why I am confused why a fork is not solving the burden of possible bug reports. Also the fork assumes its own maintenance this is the purpose of the fork.