Normal air conditioners produce a lot of heat, which is usually ventilated outside. That's more difficult to achieve deep underground.

According to Wikipedia, it's a groundwater based cooling system.

https://en.wikipedia.org/wiki/London_Underground_cooling

It's not so much that the coolers are experimental, but what they're doing with them is an experiment.

The tube did not have powered A/C until fairly recently - it was built with ambient air ventilation only.

And that A/C is only on cut and cover lines (circle etc) not deep level trains.

On deep level trains there is no good place for the trains to dump the heat.