They can tell that you used an Apple device to receive the email. They can't tell whether you actually opened it, or when, or where.

Not seeing the IP is also important for preventing linking: if you have data from other sources (your website, apps, etc.) this can link that activity to an identity for as long as your IP doesn't change. It's not perfect but that's the kind of thing advertisers like because they can see that, say, session A on a desktop computer which generated the email lead to session B on a phone which opened it and consider that all future activity linked to the session IDs from either client is the same person even when your phone moves to another network.

They’ll learn you received it, not when you opened it (as the image is downloaded either way), and won’t be able to get any metadata (ip based location, browser headers).

Gmail uses a similar technique to mask metadata, though iirc they do download the images only when the email is first read by the recipient.

The only thing they could know if its were actually delivered.

Image download would happen from a 3rd party and as soon as its received.

So actual usability is quite low. They would spam you anyway, though.