AFAIK they use it internally, there are articles on lwn.net how, the use cases is for root filesystems and containers. I'm not sure I understand what you mean by the community sentiment, there are examples of code they'd developed internally first and sent it upstream, but in all cases I remember there were no problems. What can happen in the community is e.g. how the patches are organized or if the changelogs are complete. It's of course easier to develop something internally, if it touches other subsystems or if there's enough coverage just for the new code the test/fix/deploy cycle is much flexible. Once it's supposed to go through mailinlists or convincing other maintainers to accept changes it takes longer and must stick to the development cycle. This benefits both sides in the long run.

Their culture of "works for me" attitude makes most stuff they care irrelevant to the rest of the world.

Meta and related companies have much better systems available for durability than RAID5. I would be moderately surprised if they commonly use any standard RAID level.