"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).
