does that actually matter much on anything except dirt-bike tracks, or trying to go 40mph on a horrifically bumpy track? minus some comfort advantage, of course.

like technically, sure, it's obviously true. but for performance it only really matters when you would get air time with higher mass, and the lower mass stays in contact more. commuter e-biking generally doesn't get anywhere near those speeds or bump-sizes. (trail biking: sure! I 100% believe it's a sizable consideration there)