See where you're coming from but also greatly disagree:
-If they're likely to be "serious tournament players" as you say, that means that serious tournament players judged this faction to be strong enough to pickup very early, and are succeeding heavily against other serious tournament players as well.
-These people have also had the least practice with this new faction yet are still dominating. Sure you could say the opponents have also not had a chance to practice against them, but their opponents are at least experts with their own faction vs noobs with a new faction.
-The reason this high win rate pattern of new factions is a thing is actually because of the general power creep of GW's releases. It is pretty self-evident, even if some releases are not quite as OP as others.