They weren’t mistaken for a rival cartel. It was a targeted attack on this family. They have a history of trying to stand up to the cartels, and the cartels are trying to make a statement to them.
The story I read described them as members of an offshoot church. It also used the term fundamentalist but didn’t directly imply that’s what they were.
The Church of Jesus Christ of Latter-Day Saints issued a statement, expressing sadness of the tragedy and clarifying that the deceased are not members of that church — i.e., the deceased are part of an offshoot.