Its all to do with how to grid works. Compute loads are large stable loads with ability to perform instant load ramping. The base load of the power grid is increasing (as it has done basically forever) base load is provided by slow to ramp stable large energy sources operating at heigh efficiency (nuclear, coal, geothermal etc) then u have a varying load which is the instant to instant power usage. Renewables provide unstable sources. If the renewables turn off (clouds, the wind stops etc etc) then u need to either generate more power to compensate or shed load. A compute resource can instantly shed load and sell that power they have provisioned back to the grid at a profit. Its using more power yes but it's stabilising the grid which is the big issue with renewables.

Xai is simply using industrial generators that would usually be busy making power for something else they are not increasing the net carbon output they are simply reallocating it. It does have proven impact but on the scale of global carbon emissions its fuck all.