Messaana clearly though that she matched up to Semirhage but that does not mean that she actually was her equal, just that she though she was. I think RJ aluded to the fact that some of the forsaken were better at judging their own abilities compared to their rivals and they were always at the top of the forsaken league table in the AoL. He mentioned Ishamael, Lanfear, Grendal and Semirhage as well as Sammael & Demandred.

Rahvin who was not always on top thought he could take Lanfear if he had to but again I think the evidence shows that if he had tried things may have been a lot harder that he thought. Likewise I think Mesaana may have thought she was Semirhage's equal but again she was not on the top tier list, she may have been just as accurate in estimating her abilities as Rahvin.