Here the 1840 edition dropped the subordinate conjunction that. The RLDS textual traditional has continued with this shorter reading. As discussed under Alma 27:2, that is optional for finite clauses that complement the verb see; in each instance we follow the earliest textual sources (thus retaining the that here in Alma 47:15). For a complete discussion, see under that in volume 3.
Summary: Maintain in Alma 47:15 the subordinate conjunction that after the verb see (“when they saw that they were surrounded”).