Why wouldnt a heatpump work during the winter? I.e. the entire nordic uses heatpumps..

The "entire nordic" doesn't have bombs falling on the power plants.

It's solar generation which doesn't work in winter in Ukraine, not the heat pumps. And as Russia is targeting Ukraine's energy infrastructure as their main tactic, with the goal of getting the population to accept Putin's domination to avoid freezing to death, the heat pumps would only be useful if local solar worked.