So the recipients of 2 Thessalonians 1 were there being taught Jewish truth even though Paul used his message as an encouragement to church age Christians. The Christians at Thessalonica must have there been acting as representatives of Jewish tribulation saints. To be a good dispensationalist, one must read Scripture with one's presupposed dichotomies ever in mind so that one can catch such distinctions. Dividing Scripture categorically and relegating the message to the proper age and people are key to good dispensational interpretation. Dividing Biblical history into a progression of dispensations is not unique to dispensationalists.
All theologians do that.
Having in practice rejected the typological and organic union of the two testaments that is found in Christ and His saving work, the consistent dispensationalist has instead adopted a two program, two people view of Biblical history in which the church age is a logically unnecessary parenthesis in the divine program and, from the perspective of the Old Testament prophets, a divine afterthought and adjustment. My own opinion, to use a pun, is that consistently interpreting Scripture through the rigid grid of dispensational assumptions has the potential for turning Biblical bread into theological shredded wheat. Fortunately, many dispensationalists today are mild dispensationalists who are not all that rigid when it comes to dispensational interpretation and theology and who have had little actual exposure to the classical and definitive dispensational works by men such as Darby, Chafer and Scofield where these dispensational dichotomies are rigidly pressed.