I disagree. Email has SPF and DKIM an what have you exactly because client side filtering doesn't work right. Mail gets dropped beforr the clients even get a chance to run filters.
That's not to say that requiring remote attestation or blocking third party clients entirely is proportional, but Apple should (and does) play a role in spam prevention.
SPF and DKIM are ways of signing a message, but it's still typically up to the recipient or the recipient's mail server to decide what to do with that signature. And they're only checkable on the recipient's mail server because email isn't properly end-to-end encrypted, and exposes metadata.
SPF and DKIM can be checked client side no problem, assuming your mail server doesn't mangle the received-from headers. We just generally only use them as server-side filtering.
That's not to say that requiring remote attestation or blocking third party clients entirely is proportional, but Apple should (and does) play a role in spam prevention.