Ducks won 5-2, so 7 goals were scored. That's 25 goals in 4 games - 6.25 goals per game. In today's NHL that's kind of a lot, and it is significantly more than the 4.5 GPG in the Isles/Caps series.
It isn't meaningless at all. Being the most impactful player in a playoff series, or perhaps in the NHL overall -as Backstrom has been so far - is kind of a big deal for a player.
I love how you use one game, instead of the 2-1, 4-2, or 5-4 score. Guess what? One goal is still significant, because one goal can decide a game. Maybe one goal wasn't as important in Anaheim's 5-2 win(though even that is debatable, since one of them was an empty netter), but if a game finishes 5-4 you're still looking at one goal being the difference.
As for your second sentence, that's also debatable, and you'd have a tough time proving he's having a greater impact than another player in a completely different series. What isn't debatable is that Backstrom is no longer leading the league in playoff scoring, which kind of backs up everyone's claim that it's premature to really be talking about it, because we're 4 games in. He held the lead for as long as it took four other players to complete 4 games. After tonight, someone else could be leading.