I expect this to quickly come down to another "depends on what the meaning of the word 'is' is" pissing contest. Elizondo was basically a glorified clerk, responsible for administratively managing a number of small programs, including ATTIP, within his portfolio. He was not an investigator or analyst, nor did he have subordinates who reported to him who were colocated or dedicated specifically to work ATTIP. He was a pass through for Congressionally earmarked plus up funds to Bigelow and deliverables (as defined in the Statement of Work) from Bigelow to the DoD.
The IG effectively has subpoena authority over the organization they represent/support. They have no authority over Elizondo, however. All those Elizondo has accused for defaming have, in all likelihood, already had their email accounts, notebooks, memos for record, etc., seized for review by the IG. They will also be interviewed under oath. What I anticipate they will find is a number of people, both in and out of his chain of command, who didn't like him, but proving anyone purposely tried to discredit or defame him after he left government service will be difficult. If they do find proof of that, however, I expect (and hope) they will be disciplined to greatest extent possible, especially if they lied in doing so.