The way I read the comment, I think you are correct about it being a boon and not a bane. However, it is because the request was timed at the same time as the message read, not the other way around.
It seems the commenter functionally tested an external URL being requested by Gmail, and found that the request was coincident with when the commenter opened the email. This essentially "leaks" that you've opened the email as the image URL could be unique to your email message.
This is correct. This makes unique open counts much more reliable as it guarantees that the tracking image will be fetched. Before this change, one could never be certain how many people actually opened their email in gmail because some percentage of recipients would block images which naturally blocks loading the tracking image. Typically the tracking image is unique to each individual email sent.
It seems the commenter functionally tested an external URL being requested by Gmail, and found that the request was coincident with when the commenter opened the email. This essentially "leaks" that you've opened the email as the image URL could be unique to your email message.