Yes, I suspect steroids with the early adopters of weight training. Loughborough was a center for steroid research at the time as proven in the above study and its reference to another steroid study there concerning athletic performance, as well as a newspaper advertisement for participants there. His musculature did improve markedly but studies have shown too that functional strength can improve remarkably without large visible differences, especially for distance runners doing significant mileage (remember Deano's own description of Coe as originally a scrawny 5'5" guy, and one who through heavy but limited repetitions and lots of circuit training gained a great deal of strength and a fair amount of weight from where he started. I don't have Peter Coe's book with me right now but I recall he shows Seb doing high weight squats).
He just wasn't that great a prospect until Loughborough. My own hs teammate, at a hs that developed no one, was significantly faster than him at 400/800 at 15-16 off of limited miles and soccer training.