Just referencing common jokes some nations have about other nations, the french all surrender, the Japanese are horny for school girls, the british have bad teeth, Americans are all loud... that sort of thing.
For Germans it's that you invented scat tables (glass tables) to watch your sex partner shit on you.
This is pre-internet lore, so probably formulated from German inclinations towards rationality and self-examination, sprinkled with some WW2 stuff, and some actual stuff about porn/bondage communities.
It's probably better left in the past, even if it's still a cultural reference that can be used.
I love Noddy Holders (lead singer of Slade) anecdote on on Graham Nortons show saying he was paid money in Germany multiple times to shit on a glass table while a dude was underneath.