An enclosed space can also get hotter than the "burning temperature" of whatever fuel you are using to heat it up. Because heat keeps accumulating if it has nowhere to escape to.
Edit: Welp, that was weird. Not sure what I thought was Uncanny Valley–esque about this image. Don't see it now, but I did then. Fucking weird-ass brain...
So if this were true, I know, I know, but IF it were, he didn't even get it when he typed it out. He may still have never gone back and done what she said. And also, how did she know that he would type exactly that?