Frank,

I'm guessing that they are basing their claims on the unstated
assumptions that the homeowner may already be heating with
an electric resistance furnace plus the homeowner intends to
use the portable heater for zoned heating. Turn two-thirds of
your house into an unheated icebox this winter via any method
and you will save substantially on your heating bill.

Sadly, there is very little policing of misleading advertising at
the state level and the federal level.

Gideon