The leaks do point to some scrapped Electric/Steel Pokemon in the same section where the Legendaries appeared, which is what a lot of people now speculate to have been a more likely Power Plant encounter than Volcanion (who I agree would have been a weird choice even with the premise of that locked door starting as a Mythical Encounter spot). Also the Museum Audio Guide has two descriptions of Power Plants (whether different than the one we visit or one-and-the-same) using Fire or Ground-Heat to generate electricity, Geothermal energy and the like most closesly corresponding to STEAM as an energy source. The fact that this is 2 of the 3 descriptions given is weirdly deliberate as opposed to just a bunch of different Power Plant ideas ("Here's Geothermal, here's Hydro, here's Fuel burning [don't bring this one up to Lysandre]") simply because it feels redundant otherwise in an area that only exists for flavor text or a bit of world building that most of the players likely didn't bother with for most of the game's life.
Pointing to the description in the final game (as opposed to something from the leaked early builds/docs) feels a bit "no duh" considering the premise of this speculation is that the area was intended for something and then was changed not to be, which would include scrubbing text suggesting that now-scrapped content in the hypothetical.
It's a bit narrow-minded as well to focus on the Power Plant specifically as a location type, rather the broader "here is an out of the way or unique location that has no apparent significance in gameplay despite being given a discernible interaction," to which one could point at the Celebi Shrine in Johto, the White Rock on Route 224 in Sinnoh, or the P2 Lab in Unova (which is present in BW despite no significant draw like the B2W2 Plasma Frigate docking). Kalos also has another oddly-underwhelming location in Reflection Cave (which thematically relates to Carbink and another Mythical in Diancie), which was simply the location of the Alakazite in the final game despite nothing similar corresponding to that random locked door by the Power Plant like the Ampharosite.
And the hell is "coping" about it anyway? Coping involves dealing with something difficult or trying to explain away/ignore a problem in modern internet vernacular. I don't see how the connotation of either applies to people guessing "huh, this location with an interactable block to going into it feels very odd. It probably had some purpose at one point in this game that has clear signs of cut content," with a Legendary/Mythical Pokemon being a fairly common precedent for weird locations without a seeming purpose in the game due to later event triggers.
Well putting a Door with a "Locked" prompt takes more coding effort than just making it part of the wall, so the miniscule additional effort indicates that door had some game design purpose even if only at a very early planning stage.