Realistically speaking, do we even know if he himself has a detailed idea of what the ending will be ? Seeing as he is not really adverse to soft retcon/backshadowing, I'd say he could have described the general idea and/or ultimate panel/scene but not really more than that, if he talked about it at all.
His attitude regarding questions about the end of OP suggests that he would prefer to keep it to himself, imho ; if memory serves, Oda even said he didn't want any input or opinions from anyone *not supposed to* give him any, as he wants to take full responsibility for what what could go wrong in his manga.
All in all, Toriyama being a childhood hero and inspiration to him could have made him the exception to the rule, I guess.