Unless there's a particular good reason to it's actually not too bad of an idea (with a hot water radiator loop in the home at least) to keep a fuel burning radiator loop heater around and just valve it off to not interfere on normal days. Especially if the heat pump is air source (which the mentioned swedish one isn't).
It's not that I'm against full electrification, but peak cold spell heating in excess of the heat pump capacity is among the last things we ought to "electrify by force."
We'd have massive summer overcapacity at the point where we can generally afford to "just do that". And chances are that it'd not be a resistive heater but a heat pump freezing a pool worth of water to cap the cold side temperature at just under 0C, or something the like, just because of how grid intensive the correlated load of those resistive heaters is.
It might even be cheaper to use synthetic fuel produced in summer than to 100% electrify that cold spell heating capacity. Carbon capture is expensive but there are options from e.g. domestic waste incinerators, paper making, and generally waste wood/plant matter decomposition, where hydrogen can cheaply be added to produce methane (pipes) or propane/butane (tanker trucks with liquified gas).