Minming it does NOT include nest box space. That is floor space for the birds only. Also, I believe that is the guidelines for standard size birds. It may be that the smaller birds might require less space, but I would guess not a lot less.
I too was surprised when I looked for coops online at how small they were and how many birds they SAID they would house. Small crowded coops cause stress on your birds and will apparently lead to some fighting and feather picking. Spend some time on here reading some of the old threads to pick up info from those a lot more experienced than me. Maybe your coop could be enlarged???