This is really not a correct statement beyond the fact that both are a type of Unix.

Linux is not Unix: it is not derived from AT&T Unix.

By that definition, nor is BSD. It's kind of their whole raison d'étre.

Linux 2.2 or 2.4 or so (possibly only Suse Linux) even had a kernel startup message "Unix compliance testing by UNIFIX" or something, back when Unix was considered more prestigious than Linux. It is / was by some official definition "a Unix", though not "UNIX the trademark by AT&T".

I’m fairly certain they’re referring to POSIX compatibility, not calling a Linux a Unix.

Oh damn, you are probably right.

What are the differences? I think of both as Unix-type sytems with macrokernels. I have no practical experience with BSDs.

Jeez, care to reply instead of downvoting? I would really like to know. I do keep an eye on the BSDs as a good example in some areas where Linux is bad.