And, as such, OP's claims are exactly correct.
The only way this would not be true is if GMail pulled every image in every email, even if it's not read by the recipient. Given GMail's usage of the term "proxy server" in their blog post, as well as the tests by the OP and others on this thread, this appears not to be the case.
http://marketer.com/4b3403665fea6.jpg
where that hash is used to link to my email address
It will protect you from it being as a read receipt if (and I'm not sure if this is the case, though it should be trivial to test by sending email with images served from a site you control to an email you control without opening it) Google requests the image once it has received the email.
I'm sure they built-in rate-limiting to prevent DDOSing the sender's image server...
It protects you from the guys at marketer tracking your user-agent, your ip address (which gives a rough geo-ip), the number of times you opened the email, etc. It's unclear to me whether the images could set cookies before (they probably did), but even without that, they could just use etag-tricks, or stuff like that, to track you cross-sites.
Marketers might now know when you open the message, but proxying the image prevents them from getting more precise information.
(No idea exactly how much of this and more Google does, obviously, but they put themselves in position to do it)
See how marketers are scrambling to adjust to this change:
a.) Gmail is now requesting all images from proxy servers (googleusercontent.com), which incorrectly situates users in its headquarters in Mountain View, California when images are downloaded. This impacts the ability to geo-target image content for those Gmail users who are affected by the changes. (Note: Local Maps using zip codes appended as query parameters are unaffected.)
b.) Gmail is stripping the user-agent headers from the client request, which eliminates the ability to determine the Gmail user’s device and target image content appropriately.
c.) Gmail is removing the cache-control headers from the responses, which forces the user’s images to be stored in their browser’s cache for up to a day. This only impacts live image content if a Gmail user re-opens the email after the first open.
...
http://blog.movableink.com/gmails-recent-image-handling-chan...
Basically their only avenue for now is mobile email which will soon follow in adopting this method.