It was "humor" and it's also your sincere feeling on the matter. 'it's just a joke' is what people say when they're called out on their BS.
Class 3 was chosen for harmonization with s pedelecs, very likely. But if the bike lobby was so intent on harmonization, they would've picked 15 mph for class 1/2, to match Euro regs. Speed sells, and the bike lobby figured they could get away with a higher limit. Someone at Calbike, the state advocacy group, said they got to the bill too late and would've asked for a 15 mph limit for class 1 instead. Which would make a lot more sense for mtb applications. The 28 mph figure is dumb, but maybe, being 2 mph lower, it helped avoid licensing requirements on 30 mph mopeds.
You say you care about urban mobility, but theres nothing stopping people from using Class 3s on the road, for the most part, aside from some protected bike lanes that tend to suck anyway. Maybe in the future the situation on bike paths will get sorted out, but right now it's mostly a theoretical issue and no one is getting hurt.