Really? Because I didn't think a form letter, even from royalty, was correct.
They probably hand-wrote at least a handfull for immediate family and very close friends.
You probably don't send a printed generic thank you card to the Queen when she's your grandmother.
It's not about "hey, they sent thank you cards to the people they know and sent gifts/well wishes" because that's normal and no one cares, but "hey, they even sent a card back to all random people that sent a card"