Your interpretation ignores the first paragraph of his statement. He says none of his teachers knew the meaning of the shahada. His studies were not limited to al-Arid. He studied in Makka and Madina. So he is referring to his teachers there being ignorant of the shahada too.

Furthermore, if he thought that the scholars of Arid were ignorant in knowing the shahada, i.e., many scholars who were hanbalis like him and were equally opposed to istighatha and saint veneration, what do you think he thought of non-hanbalis outside of that region? He thought they were even more ignorant and in fact he made takfir of virtually all of them.