John Carmack's arguments against building a custom XR OS at Meta

What would be the real advantage of a custom OS over a Linux distribution?

The OS does process scheduling, program management, etc. Ok, you don’t want a VR headset to run certain things slowly or crash. But some Linux distributions are battle-tested and stable, and fast, so can’t you write ordinary programs that are fast and reliable (e.g. the camera movement and passthrough use RTLinux and have a failsafe that has been formally verified or extensively tested) and that’s enough?

Based on the latter tweet in the chain, I'm wondering if Carmack is hinting that Foveated Rendering (more processing power is diverted towards the specific part of the screen you're looking at) was one advantage envisioned for it. But perhaps he's saying that he's not so sure if the performance gains from it actually justify building a custom OS instead of just overclocking the GPU along with an existing OS?