I'm sure they made their calculations but I wonder where do they expect 6MW of generated heat to go. Does the underground environment remain at the same temperature despite heating? Where is the heat going?
Heat goes up, and mine layout dictates where the air flow is drawn from; best guess is lower cooler air voids, which cycle and reach dynamically stable temp. before being drawn through the heat generating compute racks.
The heat going up part may well be assisted by the same already existing HVAC for working mine portion, it may also be utilising pre existing exhaust shafts cut like chimneys.
There is likely outside air being pumped in to the active mine portion; again mine layout and topology count for a lot here, if the mine happens to inside a mountain halfway up it's entirely possible air is being drawn in through lower drive shafts to outside air all without an excess of dedicated HVAC energy.