Originally, the carbine was invented for cavalry use, since a shorter stock is easier to handle on horseback.

I suspect the GP was referring to the transition from handguns to PDWs ("Personal Defense Weapons") in the military. This happened around 1940; the US Army began issuing the M1 Carbine to tank and vehicle crews, as it was much more effective than a .45 ACP handgun while being shorter, lighter, and easier to move around enclosed spaces than an M1 Garand (the issued rifle at that time).

The M1 Carbine and similar weapons basically evolved into submachineguns - first the M3 "Grease Gun", then the M4 Carbine once the transition from battle rifles to assault rifles happened for front-line infantry.