Note that no ancient Greek text is cited. The source for ἐπινῆν is probably someone who reconstructs this infinitive without understanding that it would be a non-standard, i.e. dialectal, form — if it exists anywhere in ancient Greek, which it doesn't.
As I mentioned, ἐπινεῖν is actually attested twice, once in Antigonus Paradoxographus, Historiarum mirabilium collectio 146.1.2, and once in Aelius Aristides, Πρὸς Πλάτωνα περὶ ῥητορικῆς (Jebb page 94, line 24).