Oliver Cowdery initially wrote “if these things are true” in the printer’s manuscript; then virtually immediately he corrected the text by supralinearly inserting the not (there is no change in the level of ink flow for the not). Theoretically, either reading will work here, although the negative if- clause works better since it implies that these things are indeed true. Without the not, the if- clause is more neutral in its expectation. In all probability the original manuscript had the not.
Elsewhere the text has two other examples of “ask if S”, where S is a clause. In those two cases, the clause itself is positively stated but the implication is negative (thus reversing the polarity):
In the first case, not all is done. And in the second one, Alma wonders whether his audience has really read those scriptures, because if they had they couldn’t have been as ignorant of the Son of God as they appear to be. (Amulek, Alma’s missionary companion, expands on this point in Alma 34:2.) Here in Moroni 10:4, the if- clause also reverses the polarity, so that the presumption is that these things are true since the if- clause is stated negatively.
Summary: Accept in Moroni 10:4 the corrected reading in 𝓟 with the not (“if these things are not true”), which implies that “these things are true”.