I take it by "rather than" you mean "and not". Am I mistaken? So, here, you say that when it is said that the Father IS NOT the Son, it is therein being said that the Father is not distinct, and that the Son is not distinct.
When a person says that Abbott IS NOT Costello, then according to you, would that mean that Abbott is not distinct, and that Costello is not distinct?
When a person says that one half of a sheet of paper IS NOT the other half of it, according to you, does that mean that the left half is not distinct, and that the right half is not distinct?