Here Oliver Cowdery initially wrote and (as an ampersand) at the beginning of the clause “they answered them nothing”. Almost immediately Oliver crossed out the ampersand and supralinearly inserted but (there is no change in the level of ink flow). The original manuscript probably read but here; the most reasonable explanation is that Oliver initially wrote and because and conjoins all the other clauses in this passage (marked above in bold). For additional evidence that Oliver sometimes miswrote but as and, see under Mosiah 12:32–33.
Summary: Follow in Alma 14:18 the corrected reading in 𝓟, “but they answered them nothing” (the probable reading in 𝓞, no longer extant here).