The MSL specification is more specific on how the knowledge-containing text should be stored. It provides the 'schema' and the semantics, based on Lisp syntax. It looks like MSL is intended to be the foundation for an API that Mimix plans on building.
Is that so? After all, XML has Schema as well, and with XML you can have task-appropriate schemas.
Look, it's all S-expressions, and Lisp is certainly better than XSLT/XPath, so, sure. But there's a whole ecosystem around XML for this sort of usage that there isn't for Lisp. OTOH, Lisp is a general purpose language that lets you build DSLs as needed, so there's that advantage.