The amount of heat generated directly by all of the energy use by humans - from fossil, nuclear or renewables - is a tiny, miniscule rounding error compared to the excess heat absorbed by the atmosphere and oceans due to CO2 and the greenhouse effect.

The heat output of every single data centre on earth isn’t even worth thinking about on the scale of the global climate system.

Absolutely, the heat itself is not the problem. I read that every day the earth receives more heat energy from the sun than was ever stored in all the fossil fuels on the planet.

What's more the problem is that we could have spent that energy on much more useful things. Even if these datacenters run on green energy that means that something else isn't. And don't forget that no energy is truly green. Generation requirement, electricity transport and metal extraction for it, maintenance, nuclear fuel extraction and refinement, all these things aren't green. We can make them green as much as possible but increasing demands on energy massively is making the point of reaching carbon neutrality much harder.