I think they’re saying a human that was not able to practice walking would not be able to walk even if their muscles were fine; like an inverted Neo waking up from the matrix.

It’s hard to imagine it being possible to test but I think they’re wrong.

That's why I'm asking to clarify what they meant. Because from observing how quickly other animals (including other apes) acquire motor skills from birth, I don't see why we should attribute walking to a practiced skill for human infants, rather than a physical sturdying of their body to sustain the activity.