It‘s not totally separate from MAPI because it inherits a large amount of the legacy baggage. A compliant implementation will need a very similar object model, in particular most of the properties, streams and encapsulated formats are also transported through EWS.

Ahh.

I wonder how it deals with things like MAPI (which still forms internals of Outlook and Exchange) not actually supporting HTML email[1] despite having a message for it (and Outlook being infamous for pushing HTML email in minds of many)

[1] If you include a HTML Email message part in a MAPI store, the MS MAPI library explodes and declares the store unreadable. You have to wrap HTML in RTF and include it in RTF message part... ¯ \ _ ಠ _ ಠ _ / ¯