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.

Does that assume no pedaling though? In my experience the pain of starts and stops dominates the joy of steady state pedaling. Presumably the 3Wh/km is free/"exercise" or some portion. Whereas the .24Wh (re-gainable w/ some loss) is all sweat and pain imo.

If I'm understanding the math, maybe that scales the regenerative range extension % by your tolerance for pedaling?

The difference is that humans (unlike motors) have pretty low max power limits.

I assume this comment in relation to the starting from a stop being unpleasant?

If it's w.r.t. effect of low max power on low cumulative generation, I agree it does seem like a little silly to arbitrage your power generation this way. But maybe the tradeoff is worth it in some circumstances in their view?

Or maybe it's just a low cost addition as other commenters say.

Let's say 130kg (80kg driver + 50kg bike) going 30km/h (e-bike limit) is ~1.25Wh.