Any port scanner launched from the LAN would give the same results.
I don't think I've stated anywhere that OSX's Port Scan is unique in this. So yes, other internally-based scanners will very likely give exactly the same result. (When I used it I wasn't to know this).
What I would take issue with is that, in Port Scan, OSX in no way warns against this. Indeed, it invites you to test any IP; you actually give it the precise IP to go away and test. So it is, in my view, Apple who are at fault. Port Scan should have recognised that I'd entered an Internet IP and warned that the result was not to be trusted. I can only hope that Apple's modified its workings and incorporated qualifiers in newer versions of OSX. I'm afraid that this has been a not untypical case of Apple simply having not provided enough information to the user.