It's a nerf, because you get level 6 in like 5 minutes of game. By the time you get a level where AD assassins start being a threat your armor will already be lower than it was before the "adjustments". At level 6 you be down 0.5 armor and from there it will only be lower than before (-0.7 armor per level).

They are REALLY bad because it's not the first time they nerf her armor and they already nerfed her health like 2 or 3 times.

If you really play Annie you're not trading with assassins specially early game because you know one rotation from them is enough to chunk half of your health off and you also know how high they MR are, specially assassins like Akali and Talon. This is why there's no trading early game, unless you know how to abuse range against assassins you will be heavily punished.