Interestingly, you can still see the other poses via glitches/modded scenarios such as seeing other players die in 2D sections while in Online play.
They're mapped to an individual bone for each pose and animated via "Bone Visibility Animations" to toggle between them when in motion, and all the 2D models for Mario share the same archive for animations regardless of what costume or state you're in.
In a sense, the sprites for "Small 2D Mario" are accounted for beyond the death pose just in case the player was still doing another animation while they were dying so that the model doesn't turn completely invisible.

prob only the death model was used when you die

Since when were these used in the game