That's just the codepath from bootstrap.sh that downloads the binaries. I don't know if it will compile and run on, say, FreeBSD. It's quite possible no one tried.
According to the talk I watched, tigerbeetle makes heavy use of io_uring on linux - which isn't part of POSIX.
Adding freebsd support should be pretty easy. If it supports darwin, it'll probably already have an implementation built on top of freebsd's kqueue. Its probably just a case of wiring it up to use kqueue when built for freebsd.
Yes, that was my impression as well when I looked at it yesterday. The monotonic_linux() bit that's quoted is platform-specific because it uses CLOCK_BOOTTIME with clock_gettime(), but that seems supported on BSD systems as well. It's probably just that no one tried to run it, and no one spent any effort on it. I can't find a single mention of "BSD" in the issue tracker.