Well, yes, that is what I mean: the glass inevitably gets hot if it touches the hot injector for a while, and heats up the herb and the air inside the chamber. If you only apply the injector on the bowl while inhaling, and then immediately remove it, this won't happen.
Heat rises with hot air or water but will spread in any direction inside solid materials (how far depends on their thermal properties) and will conduct from one object to another if they are in contact.
Someone else posted about pre-heating in the Flowerpot thread a few days ago.