Hard to say the Spitfires were not the best team in the CHL.
Spitfires lost in the first round against the Knights, who themselves lost in the second round to the OHL champion Erie Otters. The Knights were the only team to push the Otters to 7 games, and the Spitfires also went 7 games in their first round series with the Knights. Those were quite possibly the 3 best teams in the OHL that year.
Spitfires were a +9 in the round robin as you say; however Erie was a +7. Those were clearly the top 2 teams at the tournament and arguably in the CHL. So, was Windsor the best team in the CHL? Well, they did go 2-0 vs. Erie in the tournament; however Erie outshot them 70-41 over those two games (DiPietro was clearly the difference).
Windsor was the best CHL team at the tournament.
Erie was the best OHL team in the playoffs.
It's definitely possible for both to be true. And in 2017 that's what happened. Doesn't take anything away from either of these teams and their accomplishments. They are different accomplishments, but both are worthy of celebration.