They are talking about brooder size. Babies, and like we use our tubs as tha chicks grow, they get moved to more and more brooders giving them more space. Then onto Grow out pens that have large wire bottoms (1/2' if I remember). Then the birds are sold for meat. So they are not raised to be very old. They are talking about growing out space. Not breeder space.
I think you are asking about adult birds, not babies or adolescents. Adult breeders need more space.
Also, take into consideration what size bird you are going to raise and or breed.
The bigger the variety the more space each is going to need. So decide that first thing.
Are you going to raise Pharoah size, the Jumboes or are you going for the biggest Coturnix that the likes of mankind has ever seen.
Because I would but Pharoah size in .75" - Jumbo's .75-1." but the big ones no less than 1"-1.25 for breeders. But more is always better.
Dang, I went back and reread it. That's what it says, but I sure wouldn't try it and I wouldn't make them that short for the big birds either. 2x'3'- I would put about 6-8 big birds in there if they all got along. Sand boxes take up room.