That one passage sounds more to me like an excuse as to why Lor'themar does nothing about her. The only other bit that fits the redemption part is Three Sisters... but then again, not murdering your family in cold blood after planning to do so is not exactly high on my list of virtues.
I do agree that a good villain needs a reason to do what they do, but since they reduced Sylvanas's reason to not dying, it's become real hard for me to care about her in the slightest.