It isn't the BBC who said they didn't find TB's evidence "credible" - it was Dame J and the BBC's sacking of TB are purported to be in response to that very significant comment. You don't need documentary evidence to say "I have been told that you were interviewed by ...................... and what was the outcome?" If someone, as has been suggested, denies that he was ever interviewed, but then gives conflicting or evasive responses on different occasions, that would presumably make them appear less credible.
On the other hand, if it is accepted that TB is deliberately being made a scapegoat of, then presumably we might entertain the possibility that Dame J is some sort of establishment "stooge" whose comment was made deliberately in order to detract attention away from the BBC's failings - or, alternatively, that the BBC had treated her comment as more significant than she had intended.
I still say only those who have had access to the huge amount of evidence are really at all equipped to come to any sort of conclusion.