Geothermal (and airbased) pumps theoretically do not have unlimited heating capacity. For example my pump (Daikin Altherma Geo 3) has a 180 litre water tank so it can ”only” supply 180 litres hot water at 65 degrees Celsius and takes about a minute to heat two additional lites.

So if I want to quickly scald myself in a 400 litre pool at fifty degrees I can’t. But if I had a gas heater that would be possible!

Depends on how you measure unlimited. My hot water heater can pretty much indefinitely supply hot water for a single shower head with a modern water saving design. It can heat faster than 1.2 gallons/minute