Ahh, okay. That's totally different! What he's saying is that he doesn't feel like your evidence is strong enough to get a successful appeal - not that your exam grade is too good to appeal!
I'm afraid we can't tell you about the likelihood of a successful appeal. The process is too idiosyncratic for us to make an honest judgement, and we simply don't have access to the information, or the expertise, available to comment.
You've got the appeal sorted out, so all we can do from here is to wish you the best of luck! There's nothing else we can really say here, or that you can really do. Just hope for the best, and I hope you make your firm!
Don't worry about not making your firm - it's not the end of the world if you don't make it into Edinburgh. You've still got plenty of other options; your life certainly doesn't end if you don't meet the offer! Plenty of other universities with great courses that can lead to equally great futures.