I don't see anything wrong with attempting this. A significant number of people mistype/change their e-mail address, and security messages from banks can be important, so anything that catches no-longer-working e-mail addresses is better for everyone involved. And I assume a very small proportion of people try to disable tracking pixels.

But this post is entirely speculation. The author has no evidence they're basing it on tracking pixels. They're literally just guessing.

And I'm dubious that tracking pixels would be a reliable enough signal to be worth it. Doesn't Gmail download images in advance anyways? Plus, I regularly filter predictable emails or just archive them directly from my inbox based on the subject line without opening.

I'd more likely assume they have an e-mail bounce detector that just has a bug in it.