What on earth makes you think it's anonymous?

It's trivial to encode each tracking pixel with a personalized hash of some sort linking it to the intended recipient of that particular email.

This is...just how tracking pixels work.

Right. The hash is anonymous to anyone without access to the internal database of hashes at HSBC. That's how tracking pixels usually work. Sniffing the HTTP connection isn't giving you any way to de-anonymize the recipient. It's a hash, not a long-term identifier.