Based on Bill Seddon’s rather excellent notes I’ve implemented decrypting MS XML documents, though only tested it on .xlsx files protected with the default VanillaSweatshop password.
MSO new-format encrypted documents aren’t in .zip format, but instead the .zip is encrypted and bundled as a stream into a classic OLE2 structured storage container which has some other streams that describe how it’s encrypted. Unlike our own which remain as .zips with encrypted streams.