Instead of the reading of the Benedictine and earlier editions, εἰ δὲ δεῖ καὶ ὥς τινες ἐξηγοῦνται τοῦτο εἰπεῖν, the mss. Roe and Casaubon combine δει και ως into the one word δικαιως, which is probably the right reading.  Something, however, is still wanted to complete the construction, and Petrus Siculus (circ. a.d. 870) who quotes the passage in his History of the Manichees, boldly conjectures ἔστι καὶ οὕτως εἰπεῖν.  A simpler emendation would be—εἰ δὲ δικαίως τινὲς ἐξηγοῦνται, δεῖ τουτο εἰπεῖν—which both completes the construction and explains the reading δεῖ καὶ ὡς.