That Jesus knew them is apparent also from how Mark phrases the story. It does not say that Jesus saw fishermen who are called Simon and Andrew
; rather, it says that he saw Simon and Andrew.
Their names are known by him. The concluding words of Mark 1:16 explain that they were fishermen and that makes it clear why Jesus finds them here.1
16 Passing alongside the Sea of Galilee, he saw Simon and Andrew the brother of Simon casting a net into the sea, for they were fishermen.