The typesetter for the 1841 British edition switched the order of it is, thus creating a yes-no question: “for is it not written that Zenos alone spake of these things”. However, that edition did not insert a question mark after things (neither at the end of this clause nor at the end of the following clause, which also ends in things). The 1841 change in word order was probably unintentional; it was perhaps prompted by Alma’s preceding question: “how can ye disbelieve on the Son of God” (Alma 33:14). In any event, the original declarative word order (“for it is not written”) was restored in the subsequent LDS edition (in 1849). As David Calabro points out (personal communication), the yes-no question is not really possible here since there is a logical connection between the two sentences in this passage: that is, not only did Zenos speak about these things but Zenoch also did.
Summary: Maintain in Alma 33:15 the declarative word order, “for it is not written” (the reading of the earliest textual sources, including both manuscripts).