That was on-orbit refueling of hypergolic propellants, which is already done regularly on the ISS and is conceptually straightforward since you can just use bladder tanks to do the transfer without any special considerations.
What hasn't been demonstrated is on-orbit refueling with cryogenic propellants, which involve more considerations regarding thermal and pressure management. Technically the most recent Starship flight test demonstrated on-orbit transfer of cryogenic propellants (between two internal tanks), but of course doing it with docking still needs to be done.
"Demonstrated" is a strong word given what we all saw. I'm not sure there has been any document released that they actually proved that they successfully did a propellant transfer.
"On Flight 3, they did an intertank transfer of cryogens, which was successful by all accounts,"
It has admittedly been a weirdly quiet confirmation though, coming from a NASA official rather than from the usual sources (Elon/Gwynne/SpaceX official X).
What hasn't been demonstrated is on-orbit refueling with cryogenic propellants, which involve more considerations regarding thermal and pressure management. Technically the most recent Starship flight test demonstrated on-orbit transfer of cryogenic propellants (between two internal tanks), but of course doing it with docking still needs to be done.