Under the rules, it’s an illegal check to the head if the head is the principal point of contact AND the contact was avoidable. You have to meet both criteria; just because there is head contact doesn’t mean it’s illegal.
Here is the Sammy Blais suspension, which Craig Button referenced. He got two games:
https://www.nhl.com/video/blais-susp...0360/c-7240044
That hit is similar to one involving McDavid — he also got two games:
https://www.nhl.com/video/mcdavid-su...360/c-66120603
In this case I think Dube connects with Kotkaniemi’s core as much as any head contact. I also agree it may not have been avoidable — Kotkaniemi starts to accelerate away from Tkachuk right before Dube gets there.