Uh, what do you think "granatomyot" means?

What do you think the rest of it means? According to Wikipedia[1], the phrase "ruchnoy protivotankovy granatomyot" means "hand-held anti-tank grenade launcher".

[1]: https://en.wikipedia.org/wiki/Rocket-propelled_grenade

There are hand-held grenade launchers, crew served grenade launchers, etc. None of which means it's a "backronym."

Sure it does. The acronym RPG literally comes from the phrase "ruchnoy protivotankovy granatomyot", which was the name given to a series of Russian weapons in the category of portable anti-tank rocket launchers. And this name definitely pre-dates "rocket-propelled grenade", and in fact predates the weapons being rocket-propelled at all. For example, the RPG-40 is simply an anti-tank hand grenade (https://en.wikipedia.org/wiki/RPG-40), or the RPG-43 (https://en.wikipedia.org/wiki/RPG-43) and RPG-6 (https://en.wikipedia.org/wiki/RPG-6).

Actually, looking at these sources, the non-propelled versions were actually called "ruchnaya protivotankovaya granata" ("hand-held anti-tank grenade"). Still the same acronym of course, it just doesn't include the launcher. So when the launcher part was added, the phrase changed slightly but the acronym remained the same. And it was only at this point that the phrase "rocket-propelled grenade" could be used to refer to this weapon, and the phrase was presumably created to match the acronym RPG.