The court solved it is in one of the usual ways. From the decision[0]:

"Accordingly, whether plaintiff's claim to have invented e-mail is "fake" depends upon the operative definition of "e-mail". Because that definition does not have a single, objectively correct answer, the claim is incapable of being proved true or false."

Since defamation requires a false statement, and because the definition of e-mail can't be proven true or false, it can't be defamation.

[0] - https://www.documentcloud.org/documents/3987555-Gov-Uscourts...