He quotes Isidore and endorses what he says. Then (in his reply to obj 2) he says "humility, in so far as it is a virtue, conveys the notion of a praiseworthy self-abasement to the lowest place".

I agree that Aquinas doesn't say that humility involves believing yourself worse than you are. But he also doesn't appear to me to be taking a position that much resembles Lewis's, which is the point I was making.