No, that's not the issue. When, as here, the heat is coming from below, the limiting factor is the water. Once that comes to a boil, it undergoes a phase change on the inside bottom surface, rising as steam. Once above the water but below the lid (a steamer is always covered) there's no source of heat to make it hotter than 212º. A pressure cooker solves this problem by raising the phase transition point. A combi oven (or CSO) solves it by supplying heat to the whole system, not just a contact surface. A simple steamer, though, is limited to 212º.
Bear in mind, by the way, this limitation is why a double boiler works.
ETA: I took "it" to mean the steam. If you meant the pot, we're saying pretty much the same thing.
