Why are you talking about SBCL here? This is not for the benefit of SBCL, which does not target Web-Assembly, but about implementing multiple return values easily. Yes not many languages have multiple return values today, but Web Assembly should not only focus on implement only the languages that are popular today but the languages of the future.
From what I understand, it really is just Lisp, not any language with multiple return values. The issue here is Lisp semantics for multiple return values being slightly cumbersome (but by no means difficult).