Dave P is a great raconteur and these vids are pure pleasure.
A few demurs:
POSIX meant more than just software portability (Write Once Run Anywhere) . It was the feds way of specifying UNIX w/o using the protected brandname belonging to AT&T at the time. POSIX promised that buyers of hardware systems would have a choice of vendors as long as its specifications were followed as a guideline. US DOD made POSIX certification mandatory for hardware acquisitions.
People seem to think UNIX started in 1973. In fact UNIX started in 1969 (space travel game, PDP-7 and all that). In 1973 Ken Thompson introduced the UNIX timesharing system at an ACM Symposium on Operating Systems. It was Version 3 at the time.
I was privileged to see Dave Cutler present NT architecture in 1992 at an operating systems workshop by Usenix in Seattle. That crowd was NOT nice to him. I reckon he had a thick skin.
It was after that workshop that I began to wonder if UNIX would really be the once and future system.