And who told you this bulls*** now?
Seen it with my own eyes, mate
waiting for a flight at an airport is more than just sitting around, if you open your eyes
A couple of times when I waited for flights in DUS, I had seen planes that freshly landed from whatever places, unboard and re-board within 15 minutes. Admitted, the plane was not that full, a 733 out of which roughly 50 PAX came, and another 70 (including me) boarded again. I dunno where that plane came from, but all passengers were seemingly routined business flyers (considering that they all wore suits), which made a fast turnaround possible. The plane came in at roughly 0845, and by 0900 the door was closed, and pushback began. To me that is a 15 minute turnaround... dunno how you define it though.
Also in GDN (Gdansk, northern Poland), I had been on a scheduled 735 which came from DUS to fly back there, which was turned in 20 minutes, by de boarding through the rear exit, while passengers were boarding the front one. Worked only because we were told to board by seating row (so they rolled through the plane basically). As it was an outstation cleaning was not really done, but LH passengers are usually civilized people, that don't make much dirt.
PAX-on-board refuelling happened (which only means a fire truck is waiting behind the plane to be there immediately if something happens) catering on outstations also only happens on long haul, for short hops they have the sandwiches for both legs on board.
So two examples I have eyewitnessed, which as from the routine of the crew with that, seems to happen more often than you may think.
Also a friend of mine, who is a pilot in-training with LH says they are trained to do the preparations for the next leg in that timespan.
In Southwest's early days, when they sold an aircraft and went from 3 dow to 4, to fly the same schedule they increased utilisation and reduced turnaround times. They managed it, and this was partly achieved by a 15 minute turnaround on their 737-200s (same size as 737-500s LH flies). However it was no easy task and took a lot of effort from the staff so when they got more aircraft, 20-25 minutes became more common. Keep in mind they flew on average 400nm legs back in those days. Thus SWA invented the 25 minute turnaround on the 737.
Proofs my point