Does it bring more wins against skilled pri/mage combos or less lost games against pri/mage combos than the next best weapon? If yes then it is p2w.
Also talking about pvp or a specific comp (what comp/class is the person carrying SM playing in your example btw?) isn't really needed. BiS weapon only available for those who use money is p2w. Regardless if it's pve or pvp (and regardless of their rating in arena) or just a "who has the biggest gs"-contest.
Edit: Now if it was any other weapon than the best weapon in the game, I might not consider it to be p2w. Also, I don't mind p2w or coinshop at all, I enjoy it quite a lot actually.

