Element wattage for electric radiators
These are dual-fuel radiators.  Meaning they actually can use heat from two different sources, which is why the numbers aren't adding up the way you expect.
In their standard mode of operation, they work just like a traditional radiator on a hot water heating system -- water is heated in a central boiler, then pumped through radiators throughout the building.  This is what the output heat spec is referring to -- if you run near-boiling water through this radiator, it'll output that much heat (482W in your example).  In this mode, electricity is not used at all.
Where the electric element comes into play is when the central boiler is shut off -- in the summer, for example, you may not need or want nearly as much heat as the central boiler creates, but you may still want some.  So for this use case, an electric heating element is turned on.  This element could theoretically be any size, but since the use case for electric is for when only a little heat is needed, the manufacturer decided to use elements 1.8x smaller than the radiator's output when using hot water.  So, when using electricity only, your example heater will only put out 250W.  And since the electric mode only uses 250W, your 400W rated smart switch should work fine.