Well, what never ceases to amaze me are some of the lengths that some people will go to in order to fulfil their aims. What is he expecting or hoping he will achieve by claiming to be an administrator? On the surface this issue as a stand alone will probably not warrant a permanent ban but I do agree with you that at the very least he should receive a signature ban.
I think a signature ban should be administered at a bare minimum. I'm not sure what the guy gains by putting the lie in his signature, but there is no reason to try and dupe the users of the forum. I hope you have reported him?