- You can send a message to anyone with the iOS Keybase client, asking it to sign a message saying that a certain XLM address is theirs
- Your client will happily and automatically do so and add it to your Keybase profile page, no interaction needed
I base this summary on the statements "Keybase updated their iOS client to sign an attestation, as a user, that a given stellar address belongs to them, even if it does not. This is done without any user interaction" and "There is no option to remove this payment address from my Keybase profile". Did I get that right? It seems kinda weird, but given the partnership, I guess this is the way to roll that out quickly.
So the point of Keybase is tying profiles together, like HN and GitHub account, Powerdraincurrency addresses, PGP key, etc., all with cryptographic proofs. It would be pretty weird indeed if any of the Keybase clients chose to cryptographically sign a proof for a random GitHub account upon being asked to do so, no matter whether is really is your GitHub account. I can see why the author calls this a backdoor.
But what everyone expected to read is a way for Keybase to read your messages (Keybase chat) or your files (Keybase filesystem) or something. This is not the case in any way, as far as I can tell. The "backdoor" headline is somewhat clickbaity (the owner of Keybase would probably consider it slander though it's not a good PR move to actually say that), even if I see what the author means.