I guess, he meant hit-scan "kv-2-like" model of shell calculation. More like 152mm ML-20/105mm LeFH - you can't see the shell itself, but can easily say that it is coming in by sound.
Also, it is rather historical accurate - HE-shells were produced with less propellent chardge to save space for explosives inside of shell itself, so they were slower.
Historically IS-2 gun was similar to large caliber artillery shell the projectile and then propellant where separated and that is why it has a very low ROF and a very small amount of munitions.