The Athanasian Creed isn’t by Athanasius, was written in Latin around the fifth or sixth century, and isn’t accepted by the Eastern churches. Doesn’t “the historic creeds” overstate its authority?

All correct: it is a Western creed of the fifth or sixth century, traditionally but wrongly attributed to Athanasius. Its usefulness here is not its authority but its precision. It is the clearest ancient statement of the two errors to avoid, confounding the persons and dividing the substance. The same rule is implicit in the Nicene-Constantinopolitan Creed, which East and West both accept, so nothing depends on the Athanasian Creed’s standing.