I rarely want images on my mails. Won’t this method show that I access my mail on an Apple device - this leaking information that might not be leaked otherwise?
But the purpose of the proxy is to shield the end-user's IP address, and probably their user agent, too. Some email providers already do this. If you load an image from a Yahoo mailbox, for example, the reported user agent is "YahooMailProxy; https://help.yahoo.com/kb/yahoo-mail-proxy-SLN28749.html".
If it’s loaded from an Apple ip you know Bob has an Apple device.
But if you do download a tracking pixel, email marketers already know you are using an Apple device. I see the Mail Privacy Protection feature as being primarily for people who want to automatically download images while sending less PII.