No, they didn't. There is an option to have Keybase sync a [backup] copy of your private key(s) between your devices for you but the key is encrypted by you. An

No, they didn't. There is an option to have Keybase sync a [backup] copy of your private key(s) between your devices for you but the key is encrypted by you. And, none of their stuff like chat or git etc. depends on using/accessing those keys in anyway (they built out their own domain-applicable pki for that—in other words, chat doesn't use pgp). It’s just a convenience option for those who want it and it’s not the default.