... but indeed 15 songs is pretty short.
true ... and there's no important and understandable reason to play only 15 songs at this evening ...
the open time at the airport salzburg is 6:00 AM to 11:00 PM and is very close to the salzburgarena.
they finished the show at 09:23 PM and were on board of the plane within ten minutes (ca. 09:35 PM).
90 minutes later they touched down at Le Bourget airport (ca. 11:05 PM) --> see Guy's Diary
IMO there was more than enough time to play in addition at least 1 or 2 songs ... with an additional time of ca. 10-15 minutes ...