if you ask Subramanian Swamy he will say " diplomatic immunity means the the immunity against dragging him to the local court,law procedure in foreign country but now that he voluntarily submitted and committed himself to the Indian judiciary and law by guaranteeing the return of the marines to India, he no longer enjoys diplomatic immunity"
His diplomatic immunity is intact, even in the event of an act of criminality, felony etc..
Giving a pledge is no big deal against his immunity cover.