He believes the people who say a sitting president can't be indicted, and he may well be right to do so.
He's definitely right to fear an avalanche of legal actions in the wake of an impeachment. Pence would probably be a good boy, and pardon him, but I don't think Pence would be so craven as to pardon all the kids, and the other hangers-on.
Of course, were he simply to lose the 2020 election to a Dem, there'd be no impeachment, but there'd also be no pardons. The thought of that may terrify him.