And more importantly, once the pool is warm enough (or in a very hot day), doesn't it lose its cooling efficiency?

A 30C swimming pool vs a 70C GPU is plenty of temperature difference for a heat exchanger to work with. And even indoor swimming pools have horrific levels of heat loss due to evaporation. But I think the key thing here is that they've undersized the server rack - if its only offsetting 60% of the heating bill, there's a fair margin still being handled by a regular heat pump, which can cycle down in warmer weather.