This is slander of the highest order.
Strictly speaking he's not telling the truth.
Jeb and Trump have little in common.
But what McConnell is doing is saying to Washington DC that Trump isn't the Antichrist, but a Republican president like others who went before.
That's my interpretation anyway. At a time when some are talking impeachment or revolution or coup d'etat what he's doing isn't necessarily a bad thing.