There’s nothing wrong with some beliefs being part of both medieval epistemology and ancient epistemology.

Even if epistemological beliefs depended on nonepistemological matters, the definition simply includes the epistemological beliefs. Other beliefs about nonepistemological things wouldn’t be part of medieval epistemology.

The author never suggested medieval epistemologists couldn’t write about nonepistemological things. We care about the epistemological beliefs; whatever’s not epistemological just wouldn’t be part of medieval epistemology.

There’s no reason medieval epistemology can’t include beliefs that are contradictory. Maybe some people believed “X is true” and others believed “Not X.” Both “X” and “Not X” can be part of what we understand to be medieval epistemology.

This provides a reason to think that the author’s solution to defining medieval epistemology might not be workable. If we don’t know who medieval epistemologists were, then we don’t know whose beliefs we should include as part of medieval epistemology.