I would have expected DEN to be -7 or slightly higher. That offensive line was still average and will be up against arguably the best pass rush in the league, on the road. So crowd noise & altitude should be a factor here. Plus I don't see how they run the ball at all against that defense.
Add in a defense, that wasn't able to stop the Lions run game, that was abysmal last year. Now they face a surprisingly good o-line, that had success on the ground wie CJ Anderson against a stout front 7 of the Panthers. And even if Thomas can't play, the Colts are laughably thin in their secondary: Best pass rusher Anderson still out, CB1 Davis out, CB2 Butler questionable and will have missed almost 3 weeks of practice if he is able to go. Robinson after his concussion probably out, rookie Safety who started game 1 probably out, Geathers who missed week1 is practicing for about 10 days after missing weeks, several others banged up. And now they go to Denver, playing at altitude, a place where depth and a deep rotation probably is more important than anywhere else.
Add in a few extra days of preparation for Denver due to them playing their opening game on Thursday, b2b home games (so even more of an advantage because they don't have to travel) and if you believe in that stuff, revenge factor for last years loss @IND.