Yes, he's stating you can probably get away with it even though it is not allowed.

Note that in Germany, there's a tradition of charging for hotel rooms based on the number of guests and their age (with discounts given for minors). That is in contrast to standard practice in the US which is to charge a flat rate for a room regardless of the number of people staying in it (the primary restriction being the maximum occupancy).

So yeah, you'd certainly be in breach of contract if you counterfactually claim your child is underage. In the unlikely event the hotel finds out, they might charge a higher rate and/or a penalty of some sort.