Using the data you're using, both Gallagher and Tatar have done better or about as good as they did this year before, and Danault hasn't.
Danault is also slightly younger, but way less-seasoned. Couldn't it be due to a natural progression?
As for the WOWY (idk if it's what you are using), Id suggest the difference in results could be explained not solely by the varying linemates but also by th established chemistry with TT and BG. Like how Danault in 2016-2017 produced more per 60 minutes without Pacioretty and Radulov than with them. It depends.
Not every good player can play with anyone. Hall and McDavid didnt fit. Same for Josi and Subban. So what you're pointing out is probably right, but not definitively.
But you make a compelling argument. I'd like to concede that I was partly wrong in saying he wasnt the weak link, because it appears he is on an offensive pov.
But not near the the point that he's dragging is linemates down, otherwise Tatar and BG would see at least a slight drop in production, not a better/equal outcome like this year. And the defense he offers helps making this line a net positive in GF/GA. I maintain that his contributions (offensive and defensive) are those of a top-6 C.
Lastly, about league-wide offense, it was partly due to the increase in PP opportunities, something the Canadiens as a whole didnt really benefit from since the powerplay was tuuuuuuuurrible. Tatar and Gallagher included. The whole team scored a total of 31 PP goals. That's less than their best goal-scorer had EV goals alone (BGally). Atrocious. But how could you ask Danault (or anyone on the Habs) to increase his production proportionally to the rest of the league if 1. he doesnt play PP, and 2. even if he did, it would have sucked as much. Though Im starting to think that trying PD on the second-wave wouldnt have hurt considering our "top-forwards" werent doing shit either.
So, tl;dr : you are right that he's weaker offensively than his linemates, but I maintain that he's a top-6 level center based on his offensive and defensive contributions. Voila.