I have seen this quoted online as well, I forget where. But I imagine you saw it at the same place.
I can tell you wherever you saw this though, I know 100% it does not say this in the docs from the National Archives. At the Archives it details in the Marine & Army docs for the Pacific that very few Japanese died by rifle fire of any type. It actually details that almost all Japanese in the Pacific, for every island, died by either explosives (i.e. grenades, satchel charges), flame throwers, bombs & artillery, and Naval guns.
Basically the actual documents written for the Pacific at the time said they either burnt or blew up the vast majority of every Japanese killed in the Pacific. It says rifle fire was mostly for suppression to move up close enough to use grenades, satchel charges, or flame throwers.
But no I've seen that 1/3 quote, but I can't remember where.
But yeah that source is wrong on that. I don't remember seeing any mentions of M3's in the Okinawa after action reports, or in any of the weapon counts.