Since you asked, one option would be to simply include it as an alternative part in the MIME message (which email is). In fact, I would argue, that’s the correct option.
They already do with all kind of stuff and it’s not really an issue. If you keep the name consistent people quickly figure out it’s not a “real” attachment.
That allows markup for the regular html part of the mail to double as structured data for automation. I can't really decide what I prefer – this allows deduplication but gets complicated when the presentation doesn't follow the exact structure of the object you want to describe.
Yeah, they could use RDFa, which actually can represent the exact same info (they're both encodings of Linked Data). Still, this actually makes it easier for people who dislike them to filter them out of the emails.
Microformats are actually a kind of limited semantic markup, but here they're using JSON-LD, which can represent/encode any Linked Data, as far as I know (though they only support the schema.org ontology).
