Rivian's TM-B electric bike

808Wh battery and 100 miles of range. These two numbers track with each other and are roughly believable.

OTOH, with a battery this big, a generator powered by the pedals, and regen braking this thing has to be heavy. I'd expect it to weigh at least 80 lbs. More likely 100. The fact that their "specs" say nothing about weight suggests they're embarrassed about the weight.

Regen braking is how you can tell this was designed by a moron. The energy balance simply does not favor regenerative braking on a bicycle, especially a bicycle that flippantly ignores aerodynamics like this one does. A bicyclist loses roughly all of their energy to air resistance. It's not a truck. There is not substantial potential energy to be recaptured going down hills.

Not a big cyclist but is that still true for lower speed city riding (typical to flat european cycling prone countries), hillier SF, or mountain biking?

It seems obviously true to typical racing or distance scenarios. And i notice the wind even at lower speeds on e-bikes in SF.

But between their quad scenario and what I imagine as the urban car replacement scenario it doesn't seem as obvious.

Yes, it is true at all speed and under all conditions. The system simply does not have the mass that would give it a great deal of gravitational potential energy, and it reaches a power equilibrium with the air at low speeds. Example:

100kg rider at 15 kph = .24W-h kinetic energy. At this speed there is probably roughly 11N of air and rolling resistance, so the steady state power is about 3W-h per km. If you go 1km between stops, or more, the amount you can expect to gain by regeneration is extremely small. It could perhaps extend your range by 5%, generously.