Here Oliver Cowdery initially wrote believest in 𝓞, but then virtually immediately he corrected the inflectional ending est by overwriting the st with th. As explained under 1 Nephi 11:2, Oliver sometimes miswrote the ending -eth as -est. Here in Alma 32:19, the critical text will follow the corrected reading in 𝓞.
Summary: Accept in Alma 32:19 the corrected inflectional ending -eth in “than he that only believeth”.