If instead you think of SysVR4 as the first "Unix", then Amiga Unix was indeed a very early Unix. I think this is a useful distinction, because de facto most of the software interfaces we associate with "Unix" are just System V (especially R4) in a trench coat. Note that POSIX and and SysVR4 released the same year (1988); they're technically unaffiliated efforts but represent a consolidation of a bunch of competing ideas into a ... tacit compromise.
Or, being more practical, SysVR4 is the absolute oldest "Unix" you're going to have a good chance of building modern (1990-2020s) software made "for unix" on. You can get a surprising amount of mileage out of a SysVR4 distribution -- but go any older, and you'll be in for a lot of "fun"!