Last I checked, a majority (plurality?) of peaking power plants[1] burn natural gas. When demand increases enough, the base load[2] increases and more power plants are added to the grid.

The grid is not a battery; generated power in is always close to the current demand output. Any claims that crypto-mining uses "clean" power just indicates the claimant doesn't understand that power is patently fungible[3].

[1] https://en.wikipedia.org/wiki/Peaking_power_plant

[2] https://en.wikipedia.org/wiki/Base_load

[3] https://en.wikipedia.org/wiki/Fungibility