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And this

If it is wasteful depends on the need to handle spillage and the capacity to store the heat. If they for some reason can't store more then they get from resistance heaters and the need to handle spillage is great, well then resistance heaters makes sense as combined heat/waste mechanism.

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Of course they've heard of heat pumps. And in many cases a heat pump is entirely the wrong solution.

The much under-rated resistance heater has many attributes that make it ideal for this purpose.

They can be turned on and off quickly.
They're very cheap. So when they're used for hundreds of hours a year, rather than thousands, their cost per kWh is much much lower than heat pumps.
They're very reliable, and need minimal servicing.

So, a heat pump would be terribly wasteful in this situation, of being a backup to the main boiler, to be used in emergencies or at times of very very low electricity costs; and a resistance heater is perfect.