RJ answered this directly on his blog after I asked it. He said that Alivia was much stronger than Cyndane but Cyndane had much more knowledge, which balanced it out.

It's also unfair to evaluate any battle at the Cleansing - I think we only got a pinhole view of the entire event, which did last a whole day, after all. Cyndane and Alivia might have dueled and been separated by the fires of Callandor, for all we know. I don't deny that Cyndane was somehow fought off by someone or something, but those circumstances are vague.

Besides, my point was merely that skill can compensate for a strength difference, and that strength can mean less if the channeler is unskilled or unknowledgable.