Why does Les take the class to Exodus 19 at this point?
The class is taken to Exodus 19 as part of an ongoing introduction preparing for the study of Matthew and the four Gospels. Understanding the Gospels requires understanding the Old Testament, because Scripture is a progressive revelation built like building blocks, and Exodus 19 sets the stage for Christ's first coming and His dealing with Israel.
Les explains he is still in the introduction, getting everyone ready for the study of Matthew and the four Gospels. He notes there is no use studying Acts unless you understand the Gospels, and no use studying the Gospels unless you understand the Old Testament, because all of it is a progressive revealing and like building blocks. He then takes the class to Exodus 19, saying it remains more or less an introduction to the account of Christ's first coming and His dealing with the nation of Israel.