EDIT: Totally agree about the 'important mindest for startup', I had a similar eye-opening experience, working in a startup with 'cowboy' code that was actually quite good, but I had to unlearn a bit of stuff I read as a junior/mid-level developper.
It was code that was well-architected, had well-concieved data structure, and had business value, but every "professional" coder would deem bad-quality, since it had no unit test, did some stuff unconventionally, and probably would have many linter warning.