Samkhya's claim to completeness, that twenty-five tattvas account for everything real, is only credible if the list does not visibly skip obvious categories, and pada's presence is part of what makes the claim credible. A scheme with organs for speaking, grasping, excreting and procreating but nothing for moving from place to place would leave one of the most basic things a living body does unaccounted for, an omission the classical system evidently judged unacceptable, since pada sits comfortably among the other four Karmendriyas rather than being subsumed under any of them.
The completeness this reflects is functional rather than anatomical. Samkhya is not attempting to catalogue every muscle or limb involved in walking; it is asserting that "the capacity to move the body through space" is one of a fixed and final set of basic action-types, on a level with speaking, grasping, excreting and procreating, no more reducible to physiology than mahat or ahamkara are reducible to brain chemistry. This is consistent with how the system treats every other tattva: each is a metaphysical category first, with its familiar physical manifestation treated as secondary evidence of the underlying principle rather than as the principle itself.
Because pada, like the other Karmendriyas, is produced from ahamkara's sattvic aspect rather than from anything specifically leg-shaped, the tattva would in principle apply to any embodied capacity for locomotion, human or otherwise, which is part of why Samkhya's scheme was exportable to non-human contexts, animal rebirth among them, without modification.