The object of a preposition would be like this:
Avec quoi écris-tu ?
(What do you write with?)
With quoi being the object of the preposition avec (with); however when you use the preposition de you must use Ce Dont instead of quoi.
In this example:
C'est ce dont je me souviens.
(That's what I remember)
You never see the preposition de, anywhere in the clause. How is it an object of a preposition, which is not even present in the sentence? Whereas in the first example you see the preposition avec in the sentence.