They can accept incoming connections from IPV6 trough Cloudflare's proxy, but the underlying server isn't ipv6 capable:
2023-09-27T19:17:23.955421Z DEBUG activitypub_federation::activity_queue: Activity https://lemmy.fbmac.net/activities/undo/506ed9a4-bfee-472f-8249-f802639eec8d was rejected by https://lemmy.world/inbox, aborting: Request error: error sending request for url (https://lemmy.fbmac.net/u/fbmac): error trying to connect: tcp connect error: Address not available (os error 99)
lemmy.ml seems to have the same problem
After some sleep, I have to concede you this point, I enabled Cloudflare no my side. I've read a lot of criticism of Cloudflare here on Lemmy itself, but it seems to be the lesser evil.