Yeah, but I think it's a little worse than that. Dio's stand is apparently stronger than Joseph's or Holly's, since he can make those brain bud things (presumably, those are plants). It's almost like a combination of what the Joestars can do.
Worse still, Joseph doesn't know what his stand can do, but Dio might know how to control those (unknown to Joseph) aspects of his stand, like how he can control the plant aspect into making buds.