It’s an assumption to think that just because someone has refuted other wrong ideologies then that means his own beliefs are true. A deviant might rightfully refute those who are more deviant than him, but that doesn’t mean he’s automatically upon the truth in everything else.

Al-Ghazali is a good example of that: https://islamqa.info/amp/en/answers/13473">https://islamqa.info/amp/en/an...