"actually, unless i'm mistaken, miyamoto said that mario 128 would be the true successor to mario 64."
It's the context that counts. When Miyamoto said it would be the true successor he meant it would be different than SM64 because he feels the true successor to a Mario game should be different and not just an update on the same exact formula like he feels SMS was.
When most of the anti-SMS types say it will be a true successor they mean it WILL be just an update on the same formula of Mario 64 unlike SMS which they don't believe is a true successor to it because they felt it was too different with the tropical setting, water-pack, etc...
It's confusing, I know, but Miyamoto did elaborate on the comment and make his feelings about the games VERY clear in the interview.