Heating up the coolant naturally slows down the reaction.
Except that a runaway reaction isn't the problem here. The reactor reportedly scrammed as designed. As at Three Mile Island, all of this trouble is being caused by the decay heat alone.
I note that the Wikipedia illustration of a molten salt reactor design shows a pump in the cooling system, so it's not immediately clear to me that a molten salt reactor would survive the loss of emergency power reported to have occurred here. And even in the case of a design where the cooling is driven entirely by convection — with, what, giant air-cooled heat-exchangers? — there's the question of earthquake damage to the plumbing, which I suppose is the other obvious possible cause of the current problem.