In this passage, there are two supralinear insertions in the printer’s manuscript. Both are written with heavier, darker ink and were clearly inserted later, probably while proofing 𝓟 against 𝓞. (For discussion of these and several other corrections resulting from proofing these pages of 𝓟, see under Omni 1:17.) It would appear that Oliver Cowdery, the scribe here in 𝓟, accidentally skipped over the text twice here in Omni 1:6. The first insertion, a verb phrase (“should not be verified”), appears to be necessary to make sense of the text. The second insertion, the subordinate conjunction that, is also consistent with usage elsewhere; in the expression “spake ... saying”, that can optionally appear after the word saying. (See, for instance, the discussion under 1 Nephi 7:1; for further discussion of the optionality of that, see under that in volume 3.)
In fact, this whole passage in Omni 1:6 is a negative form of a nearly identical statement found in the previous book:
Thus the two insertions in Omni 1:6 are very likely readings from 𝓞.
Summary: Retain in Omni 1:6 Oliver Cowdery’s two corrections in 𝓟, which most probably are corrections to the reading in 𝓞, no longer extant here.