What makes you say that?
Ronnie had some of the best arm (bicep) and shoulder detail of any bodybuilder in history, and that is primarily the two locations that most professional bodybuilders put synthol.
His arm detail in particular makes most of the current pros look like they are, in fact the ones putting synthol in their arms, or need to diet for another 6 weeks!
He put a lot of shots in his calves in my opinion. Look closely at his calves. It's been a standard practice for a long time, especially for black bodybuilders. As you say, delts are the place to build scar tissue with injections, rear delts in particular. Almost every singe top bb builds their delts with injections nowadays, actually since at least two or three decades.
