On principle, yes. There are quirks - the COP is different for lower temperatures, and becomes 1:1 at around 5F (-15C). So preheating the air could improve the effectiveness at low temperatures, and I am guessing it might look like this:

- before: 1kWh in, 1kWh out - after: 1.9kWh in, > 1.9kWh out

However, if it was that simple, I suppose heat pump manufactures would include a pre-heating as built-in feature.

It's very likely that he reduced the energy consumption of heat pump by 50%, but at the same time he uses more than those 50% for mining and has a negative total result that is being offset by the profit from mining itself. Which probably nice for him, but not really for environment :)