Does the kingdom of heaven being within you apply to Christians today?
No. The kingdom of heaven being within you was spoken to Jews about the kingdom offer in the person of the King. That offer was rejected, and the verse should not be applied to believers today. Instead, one should look to what the apostle of the Gentiles teaches.
Les teaches that the kingdom was right in the midst of Israel in the person of the King. Had Israel accepted that offer, they could have had the King and the kingdom; it was a valid offer, but they could not see it and rejected it. Therefore, instead of trying to use this verse for us today and teaching from what Jesus told the Jews about the kingdom of heaven being within us, Les says to go look and see what the apostle of the Gentiles says about it. On the way back to Colossians, he pauses at Acts chapter 9 to explain why he calls him the apostle of the Gentiles, noting Saul of Tarsus on the way to Damascus, breathing out threatenings and slaughter.