iMessage is one of few apps that have broad permissions to execute code in response to notifications.

For other apps like Telegram; the server can send a predefined notification message.

For iMessage, when you get something even from someone outside your contacts, its daemon invokes specific code to handle the message, and its attachments.

Whilst this doesn't help if someone opens the app, it does at least change this from a zero click attack, to a one click attack.

(This is also another example of Apple not following its own app store rules. It has privileged access to frameworks.)