Interesting question though. I think a working distinction of a rocket is that it carries oxidiser as well as fuel, as opposed to using oxygen in the surrounding air as the oxidiser. It's not entirely clear from this link:
but I suspect that it didn't carry an oxidiser, as it would have seriously compromised its range (not so much of a problem for the "ballistic" V2).
The German rocket plane - Messerschmitt Me 163 - could only carry enough fuel/oxidiser for eight minutes powered flight.