I don’t think that’s true. The issue is the amount of storage available for the regen to go. With a regular hybrid you have enough battery for maybe a couple of miles on battery depending on several factors. With a plug in you have anywhere from 15 to 30+, I think the latest prius was supposed to have something like 100 miles of range. With the larger battery the regen can be more effective on the efficiency.

It’s also dependent on the strength of the regen. There is what people are calling one pedal driving when the regen is set as high as possible, the motors will immediately start slowing the vehicle as soon as you let off the accelerator. The regen is also way more effective in the city. On the highway you’d probably want it set as low as possible. I don’t yet have a vehicle with adjustable regen so I haven’t personally tested out various scenarios to figure out what is more efficient.