Linux has been effectively Unix™ for me ever since the days when Dennis Ritchie indicated that he used Linux for everything on his web page. Late 2000s at the latest, maybe early 2000s. Unix enough for Dennis should be Unix enough for anyone.
In these efforts they also tried to create successor languages to C, Alef and Limbo (for userspace), and never cared that much about anything past C89.
He has settled with Linux because that is the winner of UNIX wars, which predated Linux wars on Distrowatch.
As for the original UNIX,
"Using Unix is the computing equivalent of listening only to music by David Cassidy"
-- Rob Pike
Today? It's all Linux or Windows (of course, now also Web/SaaS). All these different Unices have fallen by the wayside in favor of Linux. Yes, some are still around but the mindshare has been in Linux for the last 20 years.
I suspect you could rip out a ton of autoconf rules and no one would notice.