> People will "verify" the picture by looking at the repost of a screenshot of the verification UI, not by verifying the original themselves.

True.

> raises the bar but could be bypassed with enough effort.

Anyone can spoof this.

Apple cannot stop spam iMessages. They can't stop someone from rendering their privileged UI inside a browser viewport. People copy and paste remote script executions from convincing captchas.

This whole provenance thing is a red herring. You agree with me, but there's truly not a single application for this that won't be exploited.