Most likely he's just lying. Right now, there is no benefit at all to saying he would do this, because that would go completely against his "The next president should choose" narrative. He would look like a massive liar and a hypocrite.
Though... Not that he isn't one already 
He doesn't have to lie. Mitch McConnell might believe every word of what he says.
Paradoxically, should he stick to such a policy he practically ensures a narrow window in which the new Senate (with a Democratic majority due to the loss of so many incumbent Senators) gets to confirm a new Justice in the time between January 3 and January 20.
The function of the Senate in confirming a Justice is to reject an extremist, corrupt, unqualified, incompetent judge or politician for the US Supreme Court -- or someone simply a crony of the President. That is how Fortas, Carswell, Bork, and Miers got rejected.
If the appointee fits the normal standards, then the Senate needs to do its job. Elections have consequences. Barack Obama got re-elected.
Messing up the judiciary in a gamble to protect a 'conservative' majority on the Supreme Court is not simply bad policy; it's bad politics!
