How could one "inverter control" convector heater be more electrically efficient than another?
You are correct. An inefficient heater would just waste power as heat which would heat the room in any case.
Due to the slow thermal response of a room most heating systems work on a slow duty cycle by thermostat. So 30% power might be full power for 30 s on and 70 s off. Due to hysteresis in the thermostat this approach would cause some oscillation of the temperature around the setpoint. And this is really the only opportunity to improve &quot;efficiency&quot;.
If some sort of dimmer controller is used on the heater and a PID (proportional-integral-derivitive) control algorithm is used then the heater could run at 30% continuously and avoid the oscillation. This might improve the efficiency a little for a given setpoint. You'd probably need to compare against the worst thermostat type to get a 7% improvement.

Figure 1. Pavel Rojtberg's improvement to his meCoffee machine gives an idea of improved control available using PID relative to a thermostat. Source: rojtbert.net.
As to what &quot;inverter&quot; means in this context you'd need to talk to their marketing people. Generally it is used to describe a device to convert DC into AC but the heater is running on an AC supply to start with so there is no need for one.