Fat tires are definitely beneficial for heavier riders and heavier bikes, which electric bikes are.
A lot of people have opinions about rolling resistance, and wind resistance. I would like to see studies done.
I think fat tires have less rolling resistance on rough roads, as the tires absorb the bumps better.
If you look at the wind resistance of a rider, fat tires only increase this slightly, and it is only significant at high speed, or riding into a head wind.