You need the “electrical” aspect to get these efficiencies. There’s a couple aspect of it that gives it a great mpg. Engines naturally have a range of rpm and load that makes it extremely fuel efficient (worse to best can be over a 20% difference). Traditional car makers tune this to be around 60 mph since that’s the typical cruising speed of the average American. With a hybrid, you can force the engine to run at this optimal performance regardless of speed. Second aspect is regenerative braking which recovers a nontrivial amount. Lastly, with a electrical motor, you can tune the acceleration curve to use more electrical motor more than the engine to reduce acceleration deficiencies. FWIW, most hybrids sold in the US (maybe globally?) are not pluggable. It has an engine, electric motor, generator, and a very very small battery pack but standard hybrids are not pluggable (they never touch an outlet).