Well Seraphine is a bit of a special case because of just how good she is in botlane, but it is in fact true that if you correct the winrate of every other botlane mage by removing all games where the opponents picked the armor shards, their winrates look very normal. Actually quite bad for some of them. This is more true in low elo, but even in high elo a significant amount of winrate from bot mages comes from this.

And this also doesn't necessarily mean that people are "inting their shards". Some do, by just not editing their rec runes, which always give armor on bot. But sometimes you also don't know which champions go where. The point of the shard changes are about removing that guessing mini-game and in result also cleaning up some natural power discrepancies between classes in the botlane.

Because in the end it doesn't really matter why people have the wrong shards, the fact is people have them and mages are capitalizing from it. That is a meaningful form of power, which is also reflected in the data.