That is true. Both pistons in a Harley engine connect to a singular crankshaft pin (arm), thus compressing, and firing both pistons simultaneously. The inefficiency in that engine design comes from the force of the vibration of both pistons firing [nearly] at the same time, rather than staggering the pistons firing cycle.

The inefficiency comes from the design need to compensate for that extra vibration force by using more metal, and building heavier parts to withstand the shock force created by the engine, thus leading to a heavier, and inefficient, slug of a motorcycle.

There are other inefficient things with the standard Harley engine, but those other design qualities don't really affect the sound of a Harley, it is mostly the single crankshaft pin. As a side note, the single crank-pin engine design does offer a couple of minor, but noteworthy benefits.