I agree that using FreeBSD as the base OS probably isn't necessary, but it does make sense if they're trying to provide an alternative to macOS, with the macOS

I agree that using FreeBSD as the base OS probably isn't necessary, but it does make sense if they're trying to provide an alternative to macOS, with the macOS userland much more closely resembling FreeBSD's than GNU's. There are BSD-styled Linux distros that could serve this purpose too, though. The problem with using almost any existing DE as a base is that Linux DEs are all inclined toward non-mac UI paradigms (most being Windows-like and a couple iPad-like), which means required changes will be numerous and deep. So much so that the benefit of not starting from scratch is dubious.