Forget the example, just understand the reasoning. If you bet the dog you are going to win a lot of your bets because of a tie. In the NHL 1-1 and 2-2 ties are quite common, 3-3 and above are not so common. Also it is based on the fact that NHL teams are like NFL teams with the lead. Once a team gets a lead past the halfway point of the game they give up a lot of potential offensive opportunities in an attempt to avoid getting caught up ice. Home teams tend to be conservative for the last 10 minutes, but road teams can be conservative from the start of the 2nd period in some cases. So the angle worked on the fact that a road team would get the first goal or maybe a 2-1 lead and then just sit on it. When NHL teams are sitting on leads the games have little chance of becoming high scoring. Just look at the playoffs, that is all due to teams playing tight once they get a lead. No odd-man offensive rushes also means few odd-man rushes given up to the opponent. This is accomplished by having one winger or the center always trailing the play at a conservative distance and having the defense make sure no one gets behind them. It costs a team a ton of opportunities to score, but serves a purpose with a lead.
This system though mostly worked because the history of the NHL has been 5-7 goals scored a game and only in the last decade since the neutral zone trap began being seen everywhere did the NHL have this 4.5 or 5 total on most games. The books were slow to adjust but now that you have to pay a big price to bet the under, they have regained enough edge to render this strategy less valuable.