Yes all onion service connections appear from 127.0.0.1 but the current v3 onions are not guess or enumerateable. If you don't publish your address you get 0 attacks ever. Have you put your onion address all over the Internet to get attacked?
When you make a HS, it is announced in the Tor's dHT. Now, it is true that the scope of the v3 dHT is limited, it is not 0.
(And more specifically, Tor is a 6-4 mixing network overlay with dHT DNS.)
This is the information I have. It's not possible for a relay to know an .onion address like it was with v2. Could you please link to something that proofs the contrary?