I won’t defend this guy, but if I were being questioned about a case and I had any doubts of where the investigation was going, I would also plead the 5th.
He is not under trial for a criminal offense. Is he still allowed to take the fifth? That is the question.
Bingo!
Some here must not recall how easy it is to set a perjury trap. This was a deposition, whether or not he ever broke the law might not be at all relevant to the larger case but a judge may require him to answer the question. His attorney might have advised him to save a lot of time and answer the question in a way that wouldn't damage him.