Not true. Dorian stated in an interview with Peter McGough in 1993 that his arms measured 21.75 inches offseason. Obviously the biceps tear put paid to that - I don't think he would have bothered measuring them after that - he probably didn't give a shit while he was beating other bodybuilders for the Olympia title with one good arm when they had two.
Here's the Dorian quote from the seminar:
"mentioned FLEX Wheeler's claim of a 23.5 inch arm at 220 lbs in contest form... Yates laughed and replied that at 295 lbs off-season his arms only barely broke the 20' inch mark (20.125'') when he finished a bicep workout. (Apparently Yates was very fastidious about measurements and records)"You may not believe the quote but to add credence to that story, the Guinness Book of Records people measured Greg Valentino's synthol filled growths live on tv... the result: 24 inches''
Anyway Colemans arms were measured at a bit over 21 inches and his arms looked a hell of a lot bigger than Dorians.
Here's an offseason pic of Dorian during his prime and he doesn't look anywhere near 21 inches, more like 19 with a pump.