Personally I don´t think that´s true at all. Ducati has always been a weird bike, with a great engine but odd handling. If he won with KTM that would be a different story, but a bike with a good engine can be a great bike if it matches his rider characteristics
I think we all agree Lorenzo is a great rider, but now he looks useless when compared to Dovizioso because his riding style is very different to the one required to be fast on a Ducati (he´s admitted himself he need to modify his riding style). The opposite is also plausible obviously, and I think that´s what happened with Stoner, both rider and bike were a good match.
When he rode a good bike he won a title, but on next season he was beaten by Pedrosa, and even when I like Pedrosa, he´s always struggled in MotoGP due to his light weight and has barely beaten any team mate
But I understand people assuming his title with Ducati made him a great, only that I disagree To me he´s a similar case to Raikonnen, a very talented driver/rider but who only perform on some seasons, while he´s lost on many others and to me the greatest always perform, to me consistency is a must to be considered one of the greats and Casey missed that part. But when he had a good season, OMG he was unstoppable