How do they determine engagement time or whether the email was deleted?
The major issue with this is going to be scaling it, with large ESP's doing 50-100 million messages a day, and 20-30% of those being opened, that's a lot of processes. This solution will probably be effective, mostly because it's going to a much smaller segment.
I still can't imagine how they would detect deletion though.
<img src="http://myserver/invisible.gif">
3. When the connection to the endless GIF dies, call that the "reading" time.
4. If a "reading" time is under some threshold call it "deleted".
That's how I did it a few years ago anyway.
https://github.com/derwiki/redirect_tracker
Like most email metrics, it's lacking -- but this is still -a little- more information, and that goes a long way in email marketing.
You might be able to have some <img> link back to your server, but even those are blocked by default.
Either way though, it wouldn't really produce a random sample, though maybe it would allow somewhat meaningful comparisons between separate email campaigns even if the absolute numbers aren't too meaningful.