Is Chaga in symbiosis with the tree it grows on?

This is a common misconception.

Chaga is classified as a tree disease in British Columbia. It infects the host tree, typically birch, and slowly consumes it. At some point in the future, the tree will die due to Chaga weakening it from the inside. This might be 5 years or 20 years but eventually, it will happen.

This is similar to the majority of medicinal mushrooms as they consume organic matter for food.