John Carmack was simultaneously working for Oculus, ID Software, and not mentioned at all here is Armadillo Aerospace where he was even doing NASA contracts all at the same time. Trying to be Buckaroo Bonzai by doing everything at once is more than most normal people could do. It was about the time Zenimax took over ID Software that John Carmack wanted to continue doing this sort of multi-tasking, but they insisted he decide who he was going to work for exclusively.... and he told them to go take a hike.
If you are an engineering who is moonlighting on multiple side projects, it can definitely get a little bit fuzzy about what stuff you develop on your own time vs. what you develop for your employer. In Carmack's case, the previous management was a whole lot more open to him pretty much doing as much as he wanted to do and however he wanted to do it as long as they got a piece of the action. ID Software got the better end of the bargain in that situation and made a whole bunch of money off of the work that John Carmack was able to perform on their behalf.
That these asshats tried to kill the goose that laid golden eggs... is what they are pissed about. Even more because that goose flew away rather than them getting to eat goose for dinner and laid some golden eggs for somebody else instead that made billions of dollars for those investors.