He notes that the Thessalonians were concerned because they thought that they were in the Day of the Lord. Then he says that this proves that the Rapture has to happen before the Tribulation. No. It is an indication -not proof- that the Rapture happens before the Day of the Lord because it is possible -not certain- that the reason for their concern was that they missed the Rapture. If the Tribulation does not coincide with the Day of the Lord, then it doesn't relate to the question.