The MWI requires fewer rules than Copenhagen, and therefore its description is smaller, and therefore it is the strictly simpler theory.
Is there anything in particular that leads you to claim Minimum Description Length is the only legitimate claimaint to the title "Occam's razor"? It was introduced much later, and the wikipedia article claims it is "a forumlation of Occam's razor".
Certainly, William of Occam wasn't dealing in terms of information compression.
If you know more Eliezer Yudkowsky facts, post them in the comments.