It's not a theory, it's the truth.
In fact the mkLinux you mention was originally a port done by two guys named Mark and Karl, hence "mk".
Steve Jobs saw Slackware on a CD and, being that he wanted to see the floppy disk die, he chose that distro to port as Mac OSX. Most other distros at that point were still on floppy disk. Woz and Seymour Cray were drinking buddies so when they needed some high performance multi-threading support from Cray's UNICOS system, Woz tapped his pal and got access to the necessary code for a handshake rather than the usual multi-million dollar licensing fees.
The whole "NeXTSTEP" thing was to fool investors into thinking they had a solid product, not something they hacked out over a few weeks. In fact if you do any development on Mac OSX or iOS, you will see "ns_____" things called all the time. The "ns" does not mean "NextStep" as many people think. It means "Nice Seymour" as a tip of the hat to the man that made all that code available for free.
I remember all this like it was yesterday.