Linus, in 2005, couldn’t have gone for murmur, from 2008.

Was there “something like murmur” in 2005 that’s cryptographically better than SHA1?

Yeah, in 2005, SHA-1 was just about the best you can do given the constraints of the time (without picking something much more esoteric, much slower, etc.). Using SHA-256 at that time would have been noticeably slower on the computers of the time, and made repo metadata take up a lot more space.