I am not so sure that is right (addressing both Rob and Dennis). What the informational note says is that listed heaters may have instructions that prohibit installing them under receptacle outlets. It does not say that all listed heaters have that instruction. More to the point, a hydronic heater is likely to be listed, even if it has no electrical parts, and its instructions might have the same restriction about the location of receptacles. Indeed, some hydronic heaters will include a fan, and that would bring an electrical listing into play.
What I would like to know, and what I think the OP is asking, is why there should be a difference. Heat is heat, regardless of its source. If it is dangerous to have a plug inserted into a receptacle that is above a heater, then it would not matter how that heater gets its energy.