"Even so, Vaccha, any physical form...(Any feeling... Any perception... Any fabrication...Any consciousness), by which one describing the Tathagata would describe him: That the Tathagata has abandoned, its root destroyed, made like a palmyra stump, deprived of the conditions of development, not destined for future arising. Freed from the classification of form (feeling...perception...fabrication...consciousness), Vaccha, the Tathagata is deep, boundless, hard to fathom, like the sea.
If the Buddha classified himself as a human being, then he would not be "Freed from the classification of form, feeling... perception... fabrication... consciousness", since a "human being" is just a combination of "form, feeling, etc" and that's it.
Since the Buddha is now free "from the classification of form, feeling, etc.", he can't be classified as a "human being".