No, that thread is right after Combat Update patch (May 2017) so the formula is correct.
Again, I will try to explain it to you, because I think you either have a problem understanding my English (I’m sorry, I’m not a native speaker) or it’s an issue on your side.
In that formula, Attack means Attack, be it physical or magical, and Defense means Defense, be it physical or magical, so you only need to ignore the part about critical because obviously spells don’t crit (if you pay attention, there is written 0 or 50% and 0 or CritAttack… this means you place 0 there or remove the part entirely) the formula will look like this for you:
Magic Damage = (skillFactor%) * (Magic Attack * 100%) * min {1, log10 ((Magic Attack / (Magic Defense + 1))^0.9 + 1)} + Additional Damage
Is it more clear now?
Also, I don’t know why you keep talking about Magic Amplification… do you really know what MagicAmp. does?
Magic Amplification amplifies (shocking!) your possible max Magic Attack by a random amount between 1 and your Magic Amp value.
Let’s say, for example, you have 200 M.Amp and 5000 Magic Attack, it means you could throw a spell with 5000 MATK + a random value between 1 and 200 (your Magic Amp), for example, 5001 or 5017 or 5034 and etc. (MAX 5000+200), so you could say it gives you an average of 100 Magic Attack.
So it doesn’t matter how much Magic Amp you have, in the Damage Formula it goes inside the Magic Attack value, it’s the same reason why there isn’t written there Min Physical and Max Physical for Physical users, the calculation is done instantly the moment you use your skill, picking up your value between min and max.
I can’t explain it any better, if you still don’t understand it, send me a PM.
I apologize to the OP and everyone else if I hijacked this thread.