In US English, I’ve always understood it to mean guys who are attached at the hip / spend a lot of time together/ to the extent they could be mistaken as being in s romantic relationship / BFFs. I felt it’s appropriate but odd used in this example. Not bad for ESL though.

That said, just because I grew up in US and my butt buddies are here doesn’t mean they’re good cofounders. I’m regularly the only entrepreneurial person in my various friend groups.