I'd say it's muscle memory. Back 20 years or so before the address bar became a search bar it was common for browsers to just successively add '.com', '.org', '

I'd say it's muscle memory. Back 20 years or so before the address bar became a search bar it was common for browsers to just successively add '.com', '.org', '.net' to an invalid address and take you to the first one that came up. It's a coincidence that modern behaviour is the same for certain domain names. If he's an experienced C++ engineer he's probably of that vintage, and has probably been in demand enough in the meantime that he hasn't had to address this "niggling" behaviour.