Seeing as no one's posted a better answer…
My interpretation of the RFCs fits with yours. I'd say Gmail's doing the wrong thing here, by the book. However, what Gmail does is de facto valid by definition. Gmail is too popular for other software not to accept however it does things, which opens the door for more software to violate the spec in the same way until it's standard practice.
Unfortunately that means there's currently no exact spec that matches reality. Luckily this question comes up in Google results now.
The original email in the question is gone, so here's another example. This is just the encoded image portion of the multipart message. Note the Content-ID header.
--089e0153807e5a346d04f1ae7c38
Content-Type: image/gif; name="blank.gif"
Content-Transfer-Encoding: base64
Content-ID: <ii_14403b4fa16783bf>
X-Attachment-Id: ii_14403b4fa16783bf
R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==
--089e0153807e5a346d04f1ae7c38--