That's more of a historical artifact. The BSDs started as a set of patches for AT&T Unix that were _distributed_ by Berkeley. Eventually they became an entire operating system. _Then_ the various BSDs that we know today (FreeBSD, OpenBSD, NetBSD, DragonflyBSD) all forked and became completely independent operating systems. For decades, FreeBSD's kernel and userland has been developed independently from the OpenBSD kernel and userland which is developed independently from NetBSD's kernel and userland, etc. You could not take an OpenBSD program and run it on FreeBSD. Even recompilation from source isn't necessarily enough since the BSDs support different syscalls.

They are completely independent operating systems with a distant shared history.

Distribution. Which is a different word than distro, with a different meaning. Like smart and smartass.

Distribution. But it’s not a Linux distribution.