Yeah, those fatbikes are just the latest iteration of the old little scooters (bromfiets) with the small win of being more quiet. I feel like the size of them and the seating arrangement should enable them to legislate fat bikes as scooters while only catching a small number of modified pushbikes that the police would likely ignore when the cyclist isn't being a nuisance.

They'll definitely gain weight, it is quite easy to tell that they aren't exerting much effort during pedal assist.