It's on par with Guaranteed Future Market Value in PCP. Where they guarantee that's how much the car will be worth... to them... not you, but sell it in way that makes it seem like bonus to you.
I used to have a non-plug in Prius years ago (2nd Gen one). As mentioned above it'd do 19mph for 1.6 miles on battery if it was fully charged (by the engine or recuperative breaking). But to do that you also had to drive like miss daisy, which would have annoyed far more people than just me (ie. like following a tractor).
The best I ever managed was 72mpg driving from Oxfordshire to the edge on London, but that was ideal condition and usually it was only high 40s mpg wise, ie. nothing special at all.
Currently I don't like hybrids, as they carry all the extra weight and complexity of both a BEV and an ICE with a extra bit of complexity to make them work together. But equally I see they have a placed for people that do a lot of miles, or who charging doesn't work. And lets be honest, if you can't use the Tesla superchargers you're into a world of fun. I'd rather in those instances it was a PHEV, and was charged than not... but in either case, surely if bought and used as it should, it's better for the planet than just going ICE.
Should the tax breaks be so generous, in my view no, but I can still see a place for them at the moment.
But in the same way as my first dig at GFMV in PCP, I think the phrase and adverts are intentionally misleading people... and should be banned. Hybrid or Plug in Hybrid and leave it at that.