It is mentioned almost summarily in the report - "Designs should be kept simple" is a phrase in there - but I think that this excessive complexity was one of the biggest factors.
This Hoare quote is relevant: "There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies."
Many embedded systems have a 'crude' OS library... and in many cases this makes them far simpler than including an RTOS. Not having seen the code here, I can't comment on this one, but just including a simple scheduler is not necessarily a bad design decision.
The the other aspects of your "Keep It Simple" answer, I fully agree with.
https://news.ycombinator.com/item?id=8374345
The Therac problem was a result of states getting out of sync and into an undesirable configuration. I think reading about the machine and then the above Carmack will cause one to see the connection.