οὔτ᾽ ἂν τῶν πρωτείων οὐδ᾽ αὖ τῶν δευτερείων ἡδονῇ μετὸν ἀληθῶς ἄν ποτε λέγοιτο:
I would expect inf μέτειναι instead of participle.
It is the impersonal usage of μέτειμι, part. neut. abs. here (cf. LSJ II).
The commentary by Bury, 1897 says that this meton is almost like a noun, so I presume that it is closer to an infinitive than to an absolute participle.