The context is that CS Lewis was a devout christian and evangelist, and is referring to spiritual desire.

I read "The Great Divorce", and interpreted it as metaphorical, in the "kingdom of heaven on earth" way. How honest work, faith, and openness are uplifting, vs getting trapped in psychological prisons.

But this quote refers to a literal afterlife. I don't know what to make of that.

It is not substantiated that all desires arise from an associated, achievable need.

I need food so I desire food and I eat to satisfy it.

I desire to have perfect love and relationships, but can’t seem to achieve it.

But if I cannot find a perfect slice of pizza, g-d exists and another world follows this one where the pizza is truly satisfying?

Believe whatever you like, but realize Faith is required for this to follow that. Logic won’t do it for you alone.

I see, the objection is specifically levelled at the mystic side of what he's saying. That's fair.

In the case that Lewis is alluding to (an unsatisfied yearning for spiritual fulfillment), the argument is tautological. If God is real in the way he imagines it, the presence of this spiritual yearning is a kind of evidence. If God is not real in the way he imagines, the presence of this spiritual yearning is evidence that the assertion is false.

Also: spiritual yearning can in fact be satisfied by spiritual practices. So the conclusion doesn't follow even if you accept his premise.