وَيَزْعُمُ أَنَّهَا تَابِعَةٌ لِلِاعْتِقَادِ
They claim that realities depend upon belief.
The position is illustrated by saying that regarding something as a substance makes it a substance, and regarding it as an accident makes it an accident; similarly, calling it eternal or originated determines its status.
A belief certainly changes the believer's mental state. It does not follow that the object believed about changes its objective nature accordingly.
The coexistence of contradictory beliefs does not establish that incompatible propositions are simultaneously true in the same respect.