Evidently this become more common (unofficially) after the first winter of the war when our troops discovered that the heavy quilted paddiing of the winter uniforms that the Chinese and North Korean soldiers wore gave them some protection from the M1 carbine 30 caliber load and even the 45ACP.
I've seen this stated many times and till recently could not figure out how clothing could stop or even slow down a .30 Carbine. Then I found a book on Body Armor of WW1. Seems that vests made of many layers of tightly woven silk could stop even a .45 colt bullet at a distance where the velocity had dropped a bit. Thinner concealable vests were only good for stopping the occasional lead pocket pistol bullet at low velocity.
The Russians took it a step further and used a thick layered silk tunic with a nickel steel breatplate over that.
During WW 2 the Russians fielded a mass produced Manganese steel breastplate that could stop the 9mm from a pistol at close range and from an MP40 at 100 yards or so. One Russian reported that a German officer had emptied a full clip into his breastplate from across a room and no bullets pentrated, but he didn't state the caliber so it may have been a .32 officers sidearm.
Anyway I figure that if the Communists had any body armor, even captured Japanese stuff, the Japanese also having developed manganese steel armor during WW2, then the unifoorm coat would have hidden it.
Also since in China silk scrap would be cheap they could make quilted linings that had some resistence to penetration if thick enough.
Another factor that could come into play. Tests using more powerful rifle cartridges against the silk armor indicated that passing through the vest somehow stabilized the bullet and slowed it so any wound would be much less severe. In intense cold such a wound from a Carbine at any great distance might not be felt as more than a punch, the victim not even realizing he had been shot for some minutes.
The Russians also developed thin titanium inserts to be placed in the inside pockets of the greatcoat, but I think that came later.