How does she know this?

That said, she has a decent idea of upper levels of male strength from facing Taim and Logain. So your general conclusion seems valid. And I think each of those circles could clearly draw more of the OP than any Forsaken. But their success or failure depended on various factors like surprise, skill, who attacked first and with what, etc.

Flinn seems to have struck hard right from the start. And he had the most success. Verin struck first too, but with a shield, and she fared the worst, losing someone after the fight began. Daigian was surprised, and had a fatally injured member of the circle to contend with while dealing with Aran'gar. I'd say she did splendidly given the circumstances. Alivia had way more strength than her opponent, and other advantages beside, but was comparatively unskilled.

If this had been the Age of Legends, these exact circles would probably have had way less problems making the Forsaken run. As such, the OP advantages of these circles allowed them to balance the much greater skill of the Forsaken... barely, in some cases.