I can definitely see it both ways. When I imagine myself as a pilot who has been on spacing/holding vectors for 30+ minutes with repeated delays and no sign of even trying to slot me in, I may very well also have fallen for the temptation of informing ATC of the upcoming fuel issue in a sarcastic way. Still unprofessional, of course, but in a different context the exact same words could be taken as a lighthearted reminder rather than a threat. (And for all my stereotypes about Germans, it may very well have been a failed attempt at humour in a strained situation.)

But yeah, from that point on ATC did seem very professional to me.