In fairness, soldiers in WWII or at least the early part, were probably more likely to know the name Garand because the popular press had covered the Johnson-Garand controversy (poorly as usual when it comes to guns), and the American Rifleman had run several articles on the new rifle. (The NRA had less than 30,000 members at the time, but the magazine probably reached more.) Other magazines had run articles also, so the name Garand was "out there" at that time.
Still, the official nomenclature was the "U.S. Rifle, Caliber .30, M1"*, and that is what it was called by rifle instructors. To cause further confusion, the Army had decided in the late 1920's to begin using a new nomenclature system to replace the old model year system. The first service rifle to be adopted after that was the "Garand" rifle, so it was called the "M1". There was also an M1 tank, M1 truck, M1 helmet, M1 carbine, M1 submachinegun, and so on.
In spite of the multiplicity of "M1's", when a soldier said, "Hand me my M1", he was talking about his rifle, not a tank or a helmet. The M1 carbine was always the "carbine" and the M1 SMG, like its M1928A1 predecessor, was a
"Tommy gun." The term "M1 carbine" was never used unless there was a need to distinguish it from the M2 carbine.
*The first nomenclature was U.S. Semiautomatic Rifle, M1" and the first 80 rifles were marked "U.S./SEMIAUTO. RIFLE/CAL. .30 M1". At serial number 81, that was changed to the familiar marking that was used to the end of production.