First, I need to state that all paints are primerless over the same type of paint. Examples; acrylic over acrylic or oil base over oil base. Also, any acrylic paint can be applied to raw drywall without many problems. The better the paint the less chance of problems.

I think that when a company states 'primer and paint in the same can' it is a marketing gimmick.

Primer has the binding agents in it.
So to keep cost down, cover paint does not have the binders.

You can look to Flood Company (flood.com)
for a product called emulsi-bond which mixes with exterior paint for use on chalky surfaces