True. That doctrine was part of the debate between wars between the traditional three branches of the Army, Infantry, Cavalry and Artillery, each of which claimed the tank should be dedicated to them.
The Infantry wanted it to be a support vehicle. They wanted a heavy, wide tracked tank with a long, high velicity gun for destroying bunkers and enemy tanks. They didn't care about speed, as they claimed it didn't need to go faster than the infantry.
The Artillery considered anyting with a cannon to be theirs, and viewed tanks as mobile artilery. They objected to any long, high velocity gun, as they burned out barrels faster. They insisted that a barrel should last at least 1,000 rounds. According Belton, it's doubtful any tanks lasted long enough in combat to ever fire a thousand rounds.
Finally there was the Cavalry. With horses obsolete, the clear answer for cavalry officers like Patton was the tank. They wanted light fast tanks to fill the cavalry role. Although they won the argument in terms of doctrine, the M4 was a compromiise; light and fast, but with a low velocity gun the artillery favored.
When I was in training they taught us the use of the tank main gun as an indirect fire weapon - in theory. I never saw anyone actually do it.