He spoke Arabic and written in Arabic too. Doesn’t make him an Arab (It does actually if it was his native tongue, unlike Persian where to the best of my knowledge, merely speaking the language is not enough).

Arabic and Persian were the languages of science and the court under the Samanids. Some of that originally-Samanid Persian+Sunni influence actually persists until today. Persian is for example still today a primary, if not the primary, language in Religious schools/madrasas in the Indian subcontinent. Doesn’t make the people there Persian.