I presume Han was told Luke left because he felt he was jeopardising their mission.
That is Vader knew Luke was there, if Luke stayed Vader could find the location of the Rebels, by Luke leaving and turning himself in he could divert Vader's attention away.
I guess it's one thing that a squadron of Stormtroopers think that a small band of Rebels may be somewhere on moon covered in Forest but it's another if you've got Vader obsessively tracking your asses.
That's the way I see it and I guess it works.