Here’s our problem, Athena:
Mr. Yonge, who provided your English translation, had only a fragmentary Greek text of paragraph 157. The Greek text you are using is more complete, so includes (just before the beginning of 158) a phrase describing a person who takes the festival seriously, λύπην καὶ φόβον καὶ ἐπιθυμίαν καὶ τἄλλα πάθη καὶ νοσήματα τῆς ψυχῆς ἐληλακότος (who has cast aside grief and fear and desire and the other passions and sicknesses of the soul). Then begins 158: ὀ δ’ ἄρτος ἄζυμος, etc., which Yonge had and which you have.
Ἄρτος is used only once here, and it is accompanied by ἄζυμος as a predicate adjective. It isn’t Philo, but Mr. Yonge, who repeats the term “unleavened bread” at the beginning of 158. A literal translation of that sentence would be And the bread is unleavened (ἄρτος [sc. ἐστὶν] ἄζυμος) either through the fact that their ancestors, when sent under divine escort into resettlement, provided themselves, under extremely hurried conditions, with unleavened dough (τὰ φυράματα ἄζυμα) from wheat flour; or because at that particular time (I’m talking about the season of Spring, in which it is seen fit to hold the festival) the grain crop is unripe, with the fields loaded with grain but not yet ready for harvest-time.
Hope all that helps.