Why isn't the cause of cystic fibrosis considered a frameshift mutation?

Cystic fibrosis is a frameshift mutation !

A frameshift mutation occurs when a protein is drastically altered because of an insertion or a deletion. 

Cystic fibrosis (CF) is a disease based on mutations in the CF transmembrane conductance regulator (CFTR) gene.Most cases of cystic fibrosis are a result of the ∆F508 mutation, which deletes the entire amino acid.

so it is a frameshift mutation !!

The answer is overmatching