OP YANBU.
They were in the wrong, and I agree with a previous poster who said that in these circumstances, the grandparents buying a present doesn't automatically mean it was done with the best intentions and that they are good grandparents as opposed to bad ones who show no interest.
I have experience of a relative who tries to control people through giving/withholding things and it's not a good place to be. Knowing that everything they do give comes with conditions and expectations can put pressure on everybody.
Okay, the OP didn't explain well in her first post, but this obsession with drip feeding invalidating any further information goes too far sometimes. It's not how real conversations work and sometimes it's not how things go on here.
I think you may have disappointed people by saying the grandparents are your parents OP, you've taken away the opportunity to be accused of MIL bashing and favouritism to your own parents.
They knew you had already bought the gift.
They knew you had asked them to stop spoiling your DD with too many presents for no reason.
They asked her what she wanted from Father Christmas, then bought the gift they knew you'd already got for her.
Most people ask children what they've asked for from Father Christmas and then check with the child's parents if it's something they can buy for the child or if it's already been bought.
They don't go out and buy it knowing that the parents have already bought it as well and then give it to the child weeks before Christmas and to me that feels as though they wanted to buy the thing they knew she wanted the most as a way to show you they won't stick to your request to buy her less.
That's not what good grandparents do. That's what controlling people do.