To me the key element is that it can run different transmission ratios: wether that's in discrete steps or a continuous ramp, that's an implementation detail. BEV (most of them) don't have that, the Prius has. When I read GP post my (surprised) impression from the "no gearbox" claim was that the Prius started as a serial hybrid. (it did not)
So the relationship between engine RPM and wheel RPM is always fixed, only torque split is varied, right? I think that's around where some confusions are coming from.
