Basically, Landsburg is positing the existence of a Platonic realm of math that is always "out-there", existing. This is a major map-territory confusion, and should be a warning to rationalists.

I agree; but interestingly, that doesn't imply that mathematical Platonism is false. I'm becoming more and more convinced that the universe is a relatively simple mathematical object, and that this universe existing is a special case of all mathematical objects existing.