Efficiency. If I'm understanding this article correctly, its comparing electric oil heaters to electric fan heaters. Both of which use electricity to create heat, both at 100% efficiency (OK, the fan in the fan heater converts some of the electricity to kinetic energy, but in the order of 1% or less in general I think). Neither are using any kind of thermodynamic cycle to extract heat more efficiently from an external source (the oil reservoir is not an external heat source), which is the only electric-powered heating method I can think of that might have greater efficiency (in terms of energy used vs heating of the room) than the direct conversion of electricity to heat that both fan heaters and electric oil heaters provide.