OP was using "read receipts" colloquially, to include "tracking images with a unique code embedded in them".
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.
Gmail seems to be proxying the images through: https://ci5.googleusercontent.com/proxy/ and my understating the polling happens when Google receives the email not when it's opened.
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.