The door stuff, including the animations of entering a door, are probably some two-way warps or something like that, or warps that activate only if the player presses some button when near them. Anyhoo, this stuff could've worked okay with the normal warps as well, so the doors are IMO just a second set of animations for the warp event, so the level creator could pick a warp type and/or set of animations for it. Let's compare this with plenty of the bridge or platform types.