> A grid mostly powered by thermal electricity is going to struggle when it gets hot.

Maybe slightly. Britannica says (https://www.britannica.com/topic/coal-utilization-122944/Coa...) that most power plants limit steam temperature to 1000 F (540 C, 813 K). The efficiency difference when ambient goes from 300K to 310 K matters... a little.

I think it's more a case of "A grid that powers a lot of peoples' air conditioners is going to struggle when it gets hot". That is, the issue is demand, not supply.