Why does Les call Matthew 24 tribulation ground?
Matthew 24 is tribulation ground because it contains no church-age doctrine and is entirely tribulation. It begins with Jesus leaving the temple, foretelling its destruction by Roman armies under Titus, and moves to the disciples' question about the sign of His coming to set up the Kingdom.
Les teaches that Matthew 24 is all tribulation and has no church-age doctrine in it whatsoever. He points out that the chapter starts with Jesus departing from the temple, and the disciples coming to show Him the temple buildings. Jesus says not one stone will be left upon another, which Les connects to the Roman siege under Titus, when the city was devastated and Herod's temple was torn down stone by stone—perhaps because soldiers had heard rumors of hidden gold and wealth between the stones. The temple was utterly destroyed, unlike Solomon's temple which burned, because Herod's was predominantly stone with gold and silver. Later, as Jesus sat on the Mount of Olives, the twelve asked privately for the sign of His coming, seeking a sign to set up the Kingdom and the end of the age. That confirms for Les that the chapter is tribulation ground, not church-age doctrine.
And Jesus went out, and departed from the temple: and his disciples came to him for to shew him the buildings of the temple.