No... That's why they had the parenthetical. The problem is, your computer probably doesn't boot the common ancestor. If you're writing UNIX-like stuff, most likely it boots macOS or Linux. If you're cool maybe it's one of the other modern BSD variants aside macOS. In practice there's a pretty low probability that your code also runs on all POSIX-compliant operating systems, and more honest/experienced people often don't kid themselves into thinking that they're seriously targeting that. Even if you believe it, you probably have some dependency somewhere that doesn't care, like Qt for example. Saying something like "Linux (or macOS, which is similar)" is a realization that you're significantly more likely to be targeting both Linux and macOS than you are to even test on BSD. And to solidify that point, note that lots of modern CI platforms don't even have great BSD support to begin with.

Of course, there is a semantic point here. macOS nominally really is UNIX, except for when someone finds out it's not actually POSIX compliant due to a bug somewhere every year or so. Still, it IS UNIX. But what people mostly run with that capability, is stuff that mostly targets Linux. So... yeah.

Of course it is true that some people really think macOS is actually Linux, but that misunderstanding is quite old by this point.

addendum: I feel like I haven't really done a good job putting my point across. What I'm really saying is, I believe most developers targeting macOS or Linux today only care about POSIX or UNIX insofar as they result in similarities between macOS and Linux. That macOS is truly UNIX makes little difference; if it happened to differ in some way, developers would happily adjust to handle it, just like they do for Linux which definitely isn't UNIX.