Although offence isn't everything, it is for a number of these players. In terms of points per game played over the past two seasons you have:
Tier 1 (1.41 and 1.40 respectively):
Kucherov and McDavid
Tier 2 (both at 1.26):
Marchand and MacKinnon
Tier 3 is a lot tougher to draw a line - there's 10 players between 1.19 and 1.10 and 8 more between 1.09 and 1.05, before there's a bit of a drop off to 1.01.
NHL.com - Stats
My initial reaction to the list was "one of the Boston players probably deserves to be on this list", and Marchand seems to have the numbers to justify being in that top 5.
A picture of the last 3 years has the same tier 1, Marchand remaining in tier 2, then bumps Crosby, Malkin, and Stamkos up to Tier 2, Kane to 2a, and MacKinnon down to tier 3.