This is how H explains ἐξ ὑπτίας in 264a : οὕτως οὗν ἔγραψε τὸν λόγον ὡς ἐξ ὑπτίας νέων ἐπ’ ἐξουσίας ἔχων καὶ ἔφ’ ἕαυτου διατιθέμενος τὸν λόγον, ὥστε παρὰ τοὺς χρὀνους ἤρξατο καὶ παρὰ φύσιν, ὡς οἱ κολυμβῶντες ὕπνιοι.
I can’t understand the following part: ἐπ’ ἐξουσίας ἔχων καὶ ἔφ’ ἕαυτου διατιθέμενος τὸν λόγον.
ἐπ’ ἐξουσίας is “at will,” “arbitrarily,” lit. “on terms of free license”
ἔφ’ ἑαυτοῦ διατιθέμενος is “arranging/organizing on his own terms," “by himself,” “independently”
τὸν λόγον (the speech) is the object of both participles.
All OK now?
οὕτως οὗν ἔγραψε τὸν λόγον ὡς ἐξ ὑπτίας νέων ἐπ’ ἐξουσίας ἔχων καὶ ἔφ’ ἕαυτου διατιθέμενος τὸν λόγον, ὥστε παρὰ τοὺς χρὀνους ἤρξατο καὶ παρὰ φύσιν, ὡς οἱ κολυμβῶντες ὕπνιοι.
he wrote the speech in such a way as swimming backwards, at his will arbitrarily arranging the parts of the speech, so that he violated the chronology and the nature [of narration], like those who swim on their backs.
ἤρξατο is important here. His organization of the speech was willful and perverse, in that “he began contrary to the temporal sequences and contrary to nature”—i.e. he didn’t begin at the beginning! (Hardly a fair criticism!)