Bernie’s position on Obama’s nominee for Merrick Garland is that he’d vote to approve him, but that if a Democrat wins in November, Obama should withdraw the nomination. Obama could then either nominate someone more liberal or wait for the President election to nominate someone.
Bernie’s position helps Garland get approved before the election, because the Republicans are gambling that a Democrat doesn’t win and nominate someone more liberal. And Bernie polls well in the general election, so the odds are good that he’d win if he was the nominee.
If a Democrat wins in November, the Republicans would probably rush to approve Garland since they know the next President would probably nominate someone more liberal. It would be a mistake to let them. Of course if a Democrat loses in November, there’s no reason for Obama to withdraw his nomination, but there’s also no chance Garland gets approved.