This is a good reason indeed, however, they’ve unfortunately left open a loophole allowing loading any DLL in the program at runtime, which in turns makes this whole “for security” reason obsolete from the get go.
Having a sandbox SDK allowing running the same code in both PC and Xbox is a good idea in itself. However this shouldn’t preclude offering supplemental means, just for PC, allowing 3rd party building more complex add-ons directly talking to the heart of the simulator and having full access to the OS features and graphics APIs.
Because honestly, I doubt any Xbox user, with a setup consisting in a Xbox controller and getting access to FS2020 via a 5$/month game pass, will spent 150$+ just for 1 airliner where only 1% of its features are within reach of a single Xbox controller…
For more complex add-ons, you need a keyboard, a mouse, you’re probably using multi-displays, specific hardware, yokes, throttles etc… even maybe remote MCDUs. I find this hardly attracting any Xbox gamer chasing Zombies with friends online, then flying FS2020 for an hour, and then taking a few laps in Forza right after…
NB: I won’t document the loophole publicly, please don’t ask, but I can say it is not using Windows supported means like process-wide DLL loading.