I think that quote was more referring to the criticism that Boucher was ranked as a late 1st or early 2nd, and the Senators should have traded down to get him.
He was referring to the intel that at least one other team (rumoured as NYR at 16) were going to select him high. So there wasn't a realistic chance to trade back and get him.
In that respect, the quote is perfectly reasonable. Most fans tend to have a confirmation bias towards players who were ranked high earlier in the process (or for a longer time) and hate when their team selects a riser or goes off the board because they both have a bias towards the players they read about more, but they also have a false perception that trading down and getting your guy works like a PS3 NHL game where the rankings are static.
Teams can usually only trade down when they have wide tiers of players who offer a similar result. Like, a team really needing a puck moving LHD, and there are 3 of them on the board, and half of the next five teams are stacked at D. Yes, they could move down a few spots and still get one of their guys.
Anaheim missed on Karlsson in 2008 because they moved down. If you go by consensus, Ottawa reached on Karlsson. The reality is that more than one team had him high.
Although, it's debatable how high Anaheim in particular were on Karlsson, they missed him by trading down and maximizing their picks. With that said, there were a lot of D on the board so their strategy may have been closer to my second example where they may have seen him as being comparable to Gardiner, which might be why they moved down.
The point is, if a player is the top player on the internal list, teams cannot just move down because a TSN's list has that player as a second rounder. That's not reality, and it doesn't work that way. That's what Mann was referring to.