I worked on a completely different hardware project within meta and while they didn't want a custom OS, they used an off the shelf rtos with the intention of modifying it and it was a shit show. They had a million justifications for why they needed it, but they had no performance tests or metrics to verify to actually justify it. They incurred a huge development overhead for no verifiable performance improvements.
None of the code they wrote couldn't have just been written as a kernel module in Linux. It would've also been so much easier due to all the documentation and general knowledge people have about Linux both within the company and outside the company.