I think the idea here is that you can take two weak sources of randomness and combine them into a stronger, single source of randomness in a homogenous way that

I think the idea here is that you can take two weak sources of randomness and combine them into a stronger, single source of randomness in a homogenous way that doesn't reveal the original weaknesses. Thus it's not adding randomness from nowhere, but composing it in a useful way. I don't actually know, but that's the only interpretation I could come up with that makes sense to me.