Interestingly enough the encounter creation rules do appear in the DMG 1. An encounter is composed of elements (monsters, hazards, traps, etc.) that "fill" up to a desired XP Budget. Encounters can be designed at any level the DM desires to achieve the wanted result, from Level - X to Level + Y. If an encounter can be of any desired level, and a trap can be an element of an encounter, or even the only element of an encounter. Then a trap can be used at any level. The idea that a trap MUST always be the same level as the PCs is preposterous as demonstrated above, and would be, in effect, contrary to the encounter design guidelines. I guess those guidelines could be considered "actual rules" also.
If the idea is to argue that the "rules" must remain inviolate, then have fun, and count me out. To remain so focused on "the actual rules" as to miss out on the actual "fun" in the game, is not my cup of tea - I'll pass. It is not a way I want to play, or run games.
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