The text is corrupt and φέγγω cannot be the assumed etymon of πῦρ.  From Byzantine sources (Et. Gudianum, Et. Magnum),  which have the same pattern as Orion, it seems likely that the real etymology in Orion was φυσάω "to blow". Cf. Et. Gudianum, pi, p. 487 (in bold the common elements): Πῦρ, φῦρ τί ἔστιν, ὅτι φυσώμενον ἐξάπτεται· ἢ παρὰ τὸ φύω, φῦρ· τί γὰρ ἐμφυέστερον πυρὸς "pūr "fire" is a *phūr, because it catches on when we blow it; or from phuō "to grow", *phūr: for what grows better than fire?" Ἐξανίσταται is synonymous with ἐξάπτεται. A hypothesis would be that the copyist mistook a lunate sigma for a gamma in a manuscript in uncials—which would imply that the error is old,—and that the resulting *φυγωμενον was later on changed by a copyist who did not understand the point. This would also account for the middle φεγγόμενον, whereas φέγγω is an activum tantum (the only instance of the passive φέγγεται in Aristophanes, Ran. 344, is not philologically secure). See πῦρ / φυσάω.