As far as the "semblance to reality", no that is real. Interfacing to a complex product is complicated. It's simpler to just get rid of driver models and layers and just avoid the entire thing. If you are DAL-A, then why not just be a foreground-background task and skip the "value added" OS layer. It's more reliable, and it's orders of magnitude simpler.
And at the end of the day, the integrator is responsible for all of it anyway. So i'd rather be responsible for 1-2 orders of magnitude less code than some unresponsive 3rd party and their crappy RTOS. And make no mistake - both vxWorks and "Integrity" are not just unresponsive, but absolute garbage. They exist because of ONLY checkbox compliance bureaucracies. You can quote all the "9's" you want - those nines were bought and paid for by the integrator, not the RTOS vendor.
Take the F-35 for example. It's one, and not the only one, that has eschewed COTS RTOS middleware.