My German is a little rusty, but I’m almost certain that this is a recommendation from Das Umweltbundesamt (national environment agency) against using hot tap water for food preparation:
Drinking warm water from the hot water pipe is only recommended to a limited extent. Because it has stayed in the pipes for longer due to the circulation in the hot water system. Circulation is necessary so that everything comes out of the tap hot enough to avoid legionella formation. On the other hand, depending on the cable material, substances can come loose. Even if the quantities are small, you should always use fresh water from the cold tap when preparing food.
My German is a little rusty, but I’m almost certain that this is a recommendation from Das Umweltbundesamt (national environment agency) against using hot tap water for food preparation:
https://www.umweltbundesamt.de/themen/trinkwasser-das-beste-lebensmittel
The middle paragraph, translated to English:
So yes, you’re right. It’s not recommended to do.