Do scholars of Islamic law who hold polytheistic beliefs and excommunicate their opponents commit kufr, or is it obligatory to establish proof against them first?

"The ruling of disbelief (kufr) upon a person is not a trivial matter. One whose Islam has been established with certainty does not lose it through doubt. This ruling is restricted to well-grounded scholars who are capable of applying Islamic rulings, while being aware of the Sunnis' principles concerning this delicate issue. Obstacles to declaring someone a disbeliever are not limited to ignorance; they also include interpretational error (ta'wil)."