That the author has risked their reputation on the claim. If you're doubting the author is legit, interrogate their professional associations with an internet search, relying on the domain name system.
Nothing about any of this is new or profound. Counterfeit documents have been around for hundreds of years.
The question you replied to wasn't "why should you believe someone who says they are behind a piece of research", it was about the usefulness of receiving an email saying it.
Their point (I assume) was that it would be illogical to worry that the report might be written and released by AI yet consider an email response as evidence against it.
If AI can create and release this report it can also hijack a real person's email or create a fake persona that pretends to be a real person.
Nothing about any of this is new or profound. Counterfeit documents have been around for hundreds of years.