The link you posted doesn’t make it seem unlikely inasmuch as it completely pillories even the notion of using Linux for avionics on both technical and economic merits, completely counter to your claim that it would “be great”. So why would it be great?
In practice most pilots use an iPad app for flight planning and navigation. No technical reason why open source hardware/software could be used for that if it had good UX (which is pretty uncommon in the Linux desktop space)
I think you and the link in OP are talking about two different things. The link talks about "safety-critical avionics" and answers mention that they use proprietary RTOS. Certainly, anything that can be done on iPad could be done on Linux, and Linux is quite common for non-real time embedded things.
