The heat emitted by the electronics will always be emitted and needs to go somewhere. If 1MWh of that heat is dumped into district heating how would that be less efficient than the 1MWh being dumped in the atmosphere to (hopefully) be reclaimed by a heat pump elsewhere?
Or, alternatively, that 1MWh could be absorbed by the already existing datacenter AC coils which could ultimately still be used to heat up district water as it cools the refrigerant. (People actually do this with swimming pools, using the coils from their AC to heat the pool).
That doesn't mean we shouldn't use the heat a data centre provides. It just means that it is not a good idea to neglect the development of energy-saving technology because the heat produced can be used somewhere else.
You said that using data centre heat is an alternative to reversible computing. This is not the case. Neglecting efficiency improvements and instead moving the data centre to a place where the heat can be used is worse than making it more efficient and using heat pumps with the saved energy at a much higher efficiency. It is even worse if the heat is dumped into the air, as istjohn said.
Also using the heat to heat homes would be even better. The two concepts aren't mutually exclusive.
In contrast, heat pumps move heat, so they can move more than the energy put into them. Even cold air (around freezing) has a lot of heat in it to siphon off.
If you're going to run 1MWh worth of compute anyway, then selling the waste heat is still a good idea. But if you weren't, a heat pump will get you more heat energy than a bank of computers with the same energy budget.
2. The point was not to replace district heating with datacenters - it is not possible, for variety of reasons, but augment the existing huge gas boilers with the datacenter to collect waste heat to render the datacenter carbon neutral.
3. Even with 400% efficiency, you will still gain if heatmpumps augmented with waste heat, as you would need far less heatpumping. You'd still need your datacenters, won't you?