Mostly just a bit of 'bro culture'. I also was told they were worried I'd 'rob their code' as I was using my personal computer instead of a company-provided one.
This guy told me the owners were worried because I could just take the code home (of course, I wouldn't delete/create the repo when arriving/leaving each day lol).
I don't know if it was really the startup founders' concerns or his paranoia, though.
After I left, I returned 6 months later to say hi (the founders discovered I was in town and asked me to go there, to their new office). So I went there, hoping this guy wasn't there, and guess what? They fired me about 2 months later after I left.
They played a trick on me, asking me what I thought about him once I came to the office. Only then, after I've politely tried to avoid shaming him while saying our working >styleI burst out in a laugh, finally telling them openly that they did the right thing. Then, they asked me why I didn't let them know about it.
I was just too naive, thinking they'd just outright reject my opinion and decided just leaving was the better thing to do for my mental health.
I was wrong: honest criticism seemed welcome by them.