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.