From the video, to my eyes anyway, it looks like Mookie's glove was is in the stands (fans are allowed to go for the ball if it's in the stands) and that he caught the ball for the out.
The theft of the ball, and the grabbing of a players mitt took place after the catch.
Looking at the play-by-play description, I only see mention of a "foul-out" and not "fan interference".
The batter was called ‘out’ anyways. But the two morons should not have interfered.................