This seems like an underestimate to me. If we use their regular season goals for and against as an estimate of their true talent (.594 Detroit; .550 Pittsburgh), Pittsburgh has a neutral-site winning percentage of 45.5%.
Of course, there is a very strong home-ice advantage in the playoffs. Since 1979-80:
Round 1 - Home Team WPCT = .576 vs Neutral-Site WPCT = .509 (Adv = .067)
Round 2 - Home Team WPCT = .548 vs Neutral-Site WPCT = .506 (Adv = .042)
Round 3 - Home Team WPCT = .546 vs Neutral-Site WPCT = .504 (Adv = .042)
Round 4 - Home Team WPCT = .563 vs Neutral-Site WPCT = .505 (Adv = .058)
Using the Round 4/Finals Home-Ice Advantage, Pittsburgh should have almost a 40% chance of winning Game 7, which means the oddsmakers are underestimating them by more than 10% - provided you don't think Pittsburgh is actually better than their GF/GA predicts. Of course, it's been 38 years since the visiting team has won Game Seven of the Stanley Cup Finals, so perhaps the home-ice advantage is larger in a deciding game than it is normally. It has only happened six times, so it's hard to know. Regardless, Pittsburgh is not the favorite to win the cup tomorrow night...