People call her anti tank because she does %max hp true dmg with every 2 autos.
tanks usually build resistance, true damage counters that
Tanks usually buy hp, %max hp counters that.
Q gives her invisibility to dodge and avoid being clicked by tanks.
and she's ranged, so, she can attack tanks before they get in range.
And she can also push them back and continue to kite and kill them.
Now, while fiora is having the same%max hp true dmg.
she is melee, she can't kite and kill tanks around. She can only avoid dmg or cc with parry.
And no steroids.
While vayne ca still get blown away by assassin, fiora usually can't in the late game.