If you have enough magic penetration to completely override the enemies magic resist, does your damage become true damage?

I was playing around with Annie and realized you can completely surpass a champions resistance if you get the upgraded boots

If you have stormsurge + shadowflame + void staff + spellslinger shoes + lvl16

You'll have 55% penetration, and 48 flat pen

So anyone that has around 110 MR or less will essentially have 0 MR

If you also buy malignance, this raises the threshold to 130 MR (though it'll only work if you're ulting them)

I ask this because true damage has a property where it isn't reduced by things like exhaust or abilities that make someone take less damage (ali ult etc).