I think people make the error of mistaking an under trained or poorly trained body part for a genuinely genetically weak body part.
If a body part is weak purely because it has previously been ignored or trained improperly, than it obviously can be improved if training is corrected.
However, if we are talking about a body part that has been trained correctly and with various methods yet still remains weak, than that's a genuine genetic weak point....and there isn't anything that is going to improve it.
Look at many BB experts/gurus/trainers etc. These guys still often have lagging parts. Does anyone think it's down to improper training considering these guys will often train each muscle using every method and technique available?
You are correct. When accessing "weak" body parts in the context of bodybuilding it matters what stage of development they are at and how they have trained. Even Arnold admitted in his first autobiography that, like most of his peers, concentrated on arms and chest. The big show pieces. It's only after he met and trained with Reg Park and really focused on his calves did they explode.
But that was the case with Arnold. Not everyone is as fortunate or genetically gifted. And lets not forget that we all start out with weak body parts. All of them. Look at Platz. Skinny arms, skinny legs, skinny everything. So why did his quads blow up to such a level that it set a whole new standard for quad development. Did he simply have more of a "mind/muscle" connection with his quads than with his arms and chest. Plaz was an especially intense and focused individual. it's highly unlikely that he did focus on his upper body as much as he did on his legs if not more so because he, like everybody else, recognized the disparity.
Barring site injects and synthol I think it was Van B who said that as the physique advances, even the weak parts, the disparity will always exist. Look at Jay. Even at 16-18 years of age he had tremendous quads but the disparity between his left and right quads were always there. Even as they both grew to world class proportions the disparity was there. Same for his arms. Even with synthol his left arm was always more developed than his right arm.