Well, you are from the US, aren't you?
Anyway, mine is just lighthearted joking; I apologize if I caused offense. The point is that, to people used to the metric system, both "pace" and "foot" sound quaint and somewhat vague measures.
Saying categorically "there is no ambiguity, a pace is three feet" sounds a little like saying "there is no ambiguity, a foot is exactly 4 palms -- that is, 16 fingers!".
Incidentally, paces (and feet) were highly standardized measures in the Roman world, much more than in medieval times. The pace was standardized not just as a "conventional measure": it was actually the (double) stride that legionnaires on the march had to keep, stretching their legs if they were short of stature!
Paces (and feet) were fairly accurate standards from what would become London to Athens, from Rome to Alexandria ... but slightly different from what Britain and its colonies eventually converged to Which adds another level of fun to the statement: from my experience to most US folks a "foot" means so unequivocally a "US foot" that there never seems to be doubt about some other possibility. So again: what sounds funny is (the appearance of) taking for granted that "foot" is a very clear, precise, universal measure, when to many people it's not.
Let me finish by apologizing again if this sounds offensive in any way -- it's not meant to be!