If they can read your notes, they can store your notes on their servers. Then when you want to delete your notes, they don’t get deleted, i.e. you don’t own them anymore.
Discussion
So if I can't melt gold down to dust, I don't own it? I can't destroy it, therefore I don't own it?
At least with a note, if someone gains access to it and tries to claim it is theirs, there is a natural mark on it that proves that claimant to be a liar and allows others to agree with me. Is that not ownership?
to own means to control fully