Why does "Many Googlers use his software internally and externally" mean he would be a good hire for Google?
From all his public complaining about failing an interview it seems Google did the right thing not hiring him, he has a massive ego and it's very possible that "writing homebrew" is less useful to google than "inverting a binary tree"
It means that he created a tool, with his skills and capabilities, that is a force multiplier for other Google engineers. This is a straight up undeniable example that his capabilities _already_ brought value to Google and their stacked deck of genius non-egostical binary tree inverters.
There's not a more pragmatic measure of whether somebody can code than a track record of a successful code project used by other coders.
Here's another way of phrasing it -- if Linus Torvalds went for an interview with Google would he have to invert a binary tree and if he failed to do so (maybe he misconstrues the question and messes up or him being Linus and just refusing) would that be a good reason to reject him? Linus also can be equally or far more egotistical than Max Howell.
I find the idea that just because someone is an excellent software engineer they are therefore guaranteed to be a good fit for a particular role at Google a bit weird
I'd say that if Linus applied to be a software engineer at Google they should be prepared to invert binary trees or do $generic_leetcode type things because that's the expectation for that role
If they applied to be Google Fellow or some other lofty position then I wouldn't expect them to need to do any coding at all in the interview
From all his public complaining about failing an interview it seems Google did the right thing not hiring him, he has a massive ego and it's very possible that "writing homebrew" is less useful to google than "inverting a binary tree"