"If I had a store, though, I would count someone walking past one way and then they other as two potential visits."

Correct. But if someone walks past really slowly, would you count that as multiple visits, because they appear in multiple frames?

'<SNIP> is quite a good proxy for "number of people present all day"'

The term to which I objected was 'Foot Traffic'. Whether something is a good proxy for <something other than foot traffic> is irrelevant.