BTW, he's not a witness in the hearing tomorrow. He doesn't care what you think of his statements.
That said, IIRC he has testified before classified House and Senate Committees on this subject under oath. They took his word.
“Chris Mellon, former Dep. Asst. SecDef for Intelligence, had a classification that permitted him to see evidence and to talk to people in the Pentagon, the services, and in the Intelligence Community about what they’ve seen and are willing to testify about.”
Having a classification and its permnissioins is nothing withou an assignment and mission with a need to know. I had the highest security clearance possible in the military and have no knowledge anything classified that was not part of my own duties. Does Mellon produce the persons he spoke to or evidence the evidence he saw? No. His statements are his only evidence. Like the say in Missouri - “show me”.