They were making a new kernel. It was not Linux and differed from Linux in a certain key area that Linus might have disagreed with someone about what back when.
I did however get the idea that they were interested in hiring people who could port Linux device drivers to their new kernel, but I don't know more because that wasn't a job I was all that interested in.
Michael Abrash was there. But he didn't seem to be all that into the OS part and more on the AR side. That was someone, I can't remember who, from Microsoft who was involved in Windows NT. And this hypothetical exchange is about that they seemed to think about it: "Are you sure a new OS is a good idea? New OSes have historically had a very low success rate." "We did it before with Windows NT and we can do it again."
But I don't entirely buy that. I don't think Windows NT would have succeeded without Microsoft's monopoly power and their willingness to use it. And Linux was free. But their new OS had neither Microsoft's monopoly power nor a low cost. Can it succeed without those things? I guess the answer was no.
I also think how to make a "new OS" thing misses the bigger picture. It's not like you make an OS and then you're done. It needs to be constantly maintained, updated for new features and new hardware. It's been 30 years, is Linux ever going to get finished? Almost done now, only a few commit left? That's not how it works! Every year is MORE commits than year before. It gets to be more work, not less, because you need to keep adding new code while maintaining the growing body of existing code.
So you've got to look not just at where the OS is now, but at the velocity and acceleration: how many commits does it get and is that going up or down. Maybe Meta's $$$ allow them to make an OS that they can use at first. Is it going to get even a fraction of people working on it that Linux has and will have? Not likely. So long term, it's going to lose.
Same reason I thought Google's "screw mainline Linux, we'll do our own Android kernel fork," strategy was doomed from the start. Seems like Google has now come to same conclusion. We'll see if the other new OS, Fuchsia, meets that same end as Meta's.