this is true. most big companies have their main software package, but also some departments do buy their own for whatever purposes. my buddy that works at spacex right now is using hypermill, and his dept has mc as well, along with NX.
I was going to say the same thing, a lot of these "top tier" more than likely have their main software that their engineers are using, probably NX, but their inhouse machine shops don't always follow, if you have a bunch of experienced MasterCam, HyperMill, programmers, machinist, whatever it may be, why would they care what CAM software is being used. Let them use what they will be successful and efficient in if it can be afforded.
I've noticed this with some larger machine shops as well, I interviewed at one locally here in MN that had a CAM room for their programmers and they believe that each programmer will be more successful with the software that they know rather than forcing them to learn something new, they no joke had multiple seats of each CAM software.