All of their previous Oz showings have been via TCM and Warner Bros., so I assumed (I know, I shouldn't assume). But that's the only logical way that I see that they would show the outtake. They wouldn't put it back in the film because it would be too jarring (the film quality is noticeable...
Note how they say "rarely seen extended musical number" and not "previously unreleased" or "newly found" which they would have said if that infamous "Jitterbug" footage had miraculously been discovered. Or anything new.
I bet that in the opening remarks given by Ben M. of TCM before the film...