C. S. Lewis can be fairly described as a Christian Inclusivist – he believed that Christianity was true, but was not willing to claim that only "Christians" would be saved.
This is different from universalism – though not his favorite doctrine, he did believe in the existence of hell, and that some would go to it:
Some will not be redeemed. There is no doctrine which I would more willingly remove from Christianity than this, if it lay in my power. But it has the full support of Scripture and, specially, of Our Lord's own words; it has always been held by Christendom; and it has the support of reason. (The Problem of Pain, chapter 8)
As opposed to Christian Exclusivism, in which saving knowledge of Jesus Christ is necessary for salvation (cf. Westminster Confession of Faith 1-1; John 14:6, Acts 4:12), Lewis (and many others) take what they consider a more hopeful approach – that some, though not knowing about Jesus Christ or not explicitly following him, will nonetheless be saved (cf. Mark 9:40, Acts 17:23–28, 2 Peter 3:9, etc.).
Lewis makes this position more explicit in Mere Christianity, using the example of "a Buddhist of good will":
There are people in other religions who are being led by God's secret influence to concentrate on those parts of their religion which are in agreement with Christianity, and who thus belong to Christ without knowing it. (Book 4, Chapter 10, "Nice People or New Men")
This is precisely what appears in this passage in The Last Battle. Aslan repudiates Tash and his wicked followers, so this isn't universalism. But despite following a false god, Aslan saves Emeth, because of his good motives and good will.