If you keep chickens like some commercial operations do, debeak them, keep light low, automatic poop handling, and some other things that we normally don't do, you can keep a lot of chickens in a small space. We on this forum normally don't do things like that.
If you have a secure area and a climate and other conditions where all you use a coop for is a safe place for them to roost, where they always have access to a larger area to roam when they are awake, there is not really anything wrong with that size coop. You feed and water outside, manage the poop, and never leave them in there when they are awake. There area some people on this forum that do that and their chickens actually lead a real good life.
The problem I see is that people think they can leave that number of chickens locked in there and they will do OK. That is a problem.
When I saw the recommended coop space per bird was 4 square feet I was a bit surprised as I have an 8x6 shed adapted as a coop and at the moment I have 27 birds in it and they are all healthy and I assume happy. They don't squabble and they all bunch up together on the roosts. I started with eight birds and about 20' of roosting space, but they all bunched up together on 3' of one perch and it looked awfully empty in there, so I got a few more, and a few more...
I think it only works because I free range them all day. I don't shut the coop doors until it's dark and they're all inside already, then at 5am my boyfriend opens the doors into the runs (8x9 on one side of the shed, 8x7 on the other) so they can come out to eat and drink. Then I let them out to free range at about 8am. In the summer they're free ranging till almost 10pm, in the winter they go back inside about 4pm.
They will be getting a second shed very soon and I don't think I'm going to bother building a run on that one as I prefer them to be free ranging.
So basically 28 sq. ft. for both the coop AND run??? Or just for the coop housing?? Even for the housing, that would be SUPER tight. But as was already mentioned, many who sell coops advertise them based on commercial poultry house standards - chickens crammed together with no consideration for comfort/humane conditions. Thankfully, most BYCers don't want those same conditions for their backyard or farm flocks.