The player ranked 91st in scoring last season, which means he's the most productive 2nd liner in the league had 50 points. That doesn't scream elite player who can produce consistently in standalone.
It really sounds like when you say top-6 you really mean 1st liner, like Plekanec who is an average to below average 1st liner.
I understand where you are coming from there are 180 top 6 forwards in the league and 109th in points should automatically makes you a top 6. I never look strictly at points. I'm sure their are some 3rd line players who amass more points than some 2nd line players. I could very well see a guy finish with 30 points who is physical, Strong along the boards, wears opponents down, and who is defensively sound as a Top 6; Depending on how his other linemates fared. Is there a magic number the members of the top two trios need to hit collectively? Or is dominating an opponent with possesion, while winning the +- war enough?
While being 109th in points should put you firmly in the top 6 mix there is much more to it than that.