I vaguely recall (assuming no bit rot) that IBM was also involved and
they refused to release their portion under FRAND terms, leading Bob
to write X.
Right, I remember that was mixed up in it. CMU (Andy "My hand is in your pocket" Carnegie) was willing to abide by the IBM terms and MIT (Paul "You gotta be kidding me" Gray) was not. So IBM got the IP from Andrew, while Athena was FOSS. IBM would eventually Open Source most of it. But originally, it was all private to them.