Let me start with a disclaimer - I've not seen a PK in person, so I can't vouch for the properties of the apron pocket for carrying concealed.
However, as a police officer, I've had experience with various garments with built-in holsters. Personally, they've usually made me a tad uncomfortable. If I'm going to carry a weapon, I prefer it snug and firm against my body somewhere. It strikes me that carrying something as heavy as a pistol in an apron pocket would encourage it to swing about with each step.
I have a vest I sometimes wear for work. It has a holster built into the lining. It holds the pistol quite well, but I am acutely aware of the pistol's movement with each step. I find myself trying to strategically place my forearm against it, or on some occasions, alter the cadence of my walk to reduce the swinging effect.
There's not much point in carrying a concealed weapon, if your efforts to conceal it actually call attention to it.

PK appears to make a very fine garment. However, I hope that this inner pocket is not the only reason for picking that particular style.