If you prowl through the specification a bit further you'll find that at
least one of the defined subtypes of message is explicitly defined to be
something other than an RFC822 message: message/external-body. In addition,
a message/partial is explicitly _not_ a complete RFC822 message and it
cannot be acted on as such. As such, any conformant MTA or gateway must
treat subtypes other than RFC822 as something other than a complete
RFC822 message, especially when the subtype isn't recognized.
The wording you refer to is unfortunate and is on my list of planned
corrections for the next draft. It dates back to the days when there were
implicit subtype defaults for each type. This concept has been removed from
subsequent versions of MIME.
Ned