I was mixed up by the translation provided by Perseus, which is correct in meaning but construed a bit differently: " the poetry of the sane man vanishes into nothingness before
that of the inspired madmen".
ἀτελὴς αὐτός τε καὶ ἡ ποίησις ὑπὸ τη̂ς τω̂ν μαινομένων ἡ του̂ σωφρονου̂ντος ἠφανίσθη
But for exercise's sake, would it be possible to construct αὐτός with ἠφανίσθη as well? Something like "uninitiated, he is obliterated along with his poetry by that of the possessed". What disturbs me is that to it seems strange to me if two elements linked by τε καὶ don't share the same verb here.
ἠφανίσθη is singular of course, but it's not seldom that Greek words are attracted to another case or number a bit like this. But would something like αὐτός τε καὶ ἡ ποίησις αὐτοῦ
Your points about "sane" and "insane" must be correct, my attempt at a translation were just trying to get to the grammatical point.