I'm not sure of the context. The form with which I'm familiar is "boots on the ground" and it's meant to mean more generally "in theater" as in contrast with doing support work somewhere back in NorthCom or Pacom. OTOH, even in theater, the number of "boots on the ground" doesn't directly translate to "number of people out in the field."

I expect the reporters borrowed this term from military people and have run with it.