Hello,
Is it possible that a customer can leave a testimonal without logging in if they have clicked the link from the email?
This is not possible as they might as well go there from the site. This is primarily done for preventing spam testimonials. The spammers will know the direct link to the input form.
The only other way around this I can see is not only to include the order ID but a unique ID for the invitation. This unique ID can't be guessed easily either by spammers.