Underfloor heating in the bathroom: More pipes instead of a radiator?

Nonsense, the statement is that normally you should create a professional room-wise heating load calculation. If the floor is then not enough, go into the walls with the pipes. And give the energy consultant a few basic data points.

and the underfloor heating may no longer be laid as closely as before.

With all due respect, that is nonsense. But yes, closer than 5cm is not possible either. And 5 cm spacing for (randomly estimated) 6-10 sqm for a bathroom is still too little. So the statement that an additional heat source is necessary can definitely be correct. Ideally, as many have already suggested, via wall heating.