I agree, there's no way to be absolutely certain that the Mario Bros are human. Although, I personally believe that both they and Il Piantissimo are. I seem to remember Il Piantissimo having some sort of obsession with Mario (but I might be mistaken), so perhaps he worked out some way to imitate him. Magic spell? Devices in his shoes? Who knows.
One thing I've noticed is that the Mushroom World seems to be filled with gifted individuals: the Mario Bros, Wario and Waluigi, Koopa the Quick, ect. Perhaps Il Piantissimo is just another one of these individuals...who just happens to be a little bit nuts. (easy way out, I know)
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"The bigger they are, the harder I fight."