IIRC one of the main reasons to split is because Vim is a one-person effort, they don't take PR and they didn't want the drastic change, so Neovim had to be a fork because there's no way to convince Vim author to take changes
That's not accurate. It's also pruning from the resulting list the commit authors for which GitHub is unable to associate a GitHub account with a given contributor; it simply doesn't attempt to represent contributors who are not (known) GitHub users.
(That isn't to say there's any significance here to Vim; the Vim repo does genuinely obscure patch authors by not using the appropriate fields to capture that information—most commits, going by Git metadata, are attributed to Bram using that method. But the statement "all GitHub's doing is looking at commit authors", strictly speaking, is not true.)
Well, the two projects don't follow the same development model so comparing (artificially capped) GitHub-centric lists is not exactly conclusive. One has to dig a little bit deeper.
In what way is it 'GitHub-centric' or 'artificially capped' though?
If I did it with `git shortlog -s` it's also going to omit the contributors from the .. I don't even know what centric way you say vim did it, and over-report commits for those who were allowed to have their names recorded properly but against commits actually authored by someone else.
It's just totally non-standard against the grain git usage.