Yes but the "progressivity" is harsh. Anything over 500 K per kid is already at 30% of taxation and anything over 1.7m is at 45%.
Also for one grandma, the n for the number of kids tends to not tend towards infinity but more towards something like 2.
So although you're right that it's not 45% on 20 m EUR (actual painting seems to be worth a bit more than that but it's to simply) but it's close. Because with two kids and the 100 K EUR excluded, you're still quickly at insane brackets rates.
For one kid I have the actual rate (I now did the computation) at 43% (instead of 45%) and for two kids I have the actual rate at 41.6%.
I understand that you're technically right to talk about n kids but it's a bit disingenuous to talk about "progressivity" of the tax rate when the effective tax rate is likely going to be 41.6% or 43% instead of the 45% I took as a shortcut.
And at 90 years old sadly the inheritance planning ain't going to be very effective. Even her kids are likely already 65 to 70 y/o, so their own inheritance planning won't allow many donation every 15 years.
It's still very much:
(french IRS): "Thank you grandma!".