If this was correct, you wouldn’t be able to read those messages with remote content loading disabled or when in airplane mode. It’s pretty uncommon for me to get messages where that’s the case, and those are almost always marketing spam so, as they say, nothing of value is lost.

Apple’s private loading feature also shows how that could be fixed: the mail server can retrieve the referenced content once and save it so you’d always know what was served at the time the message was sent.