That is (τουτο δε εστιν). "An explanatory correction" (Denney). The δε should not be ignored. Instead of saying that he had a spiritual gift for them, he wishes to add that they also have one for him.

That I with you may be comforted (συνπαρακληθηνα εν υμιν). "My being comforted in you (εν υμιν) together (συν-) with you," a mutual blessing to each party (you and me).