Every electronic device is 100% efficient after the electricity has already been generated and delivered, sure, but a bunch of efficiency losses occurred before that. If you're comparing methane burned on-site in a furnace to methane burned at a power plant, transmitted to the site as electricity, and then used for electric resistance heating, burning on-site is gonna be better even if the furnace loses more heat up the chimney than the power plant does.

Also, a heat pump is "300%-500% efficient" in the sense that it moves 3x-5x as much heat as it uses. See also: https://en.wikipedia.org/wiki/Coefficient_of_performance