Given the nature of the Sandman comics it could be interpreted in two ways, both can be equally valid.
The First Perspective
Given the nature of the Sandman comics, we are often spoken to by characters in the series through the Fourth Wall; where beings such as Lucifer or the Presence speak to the readers as well as the characters in the story. These characters exist on more than one layer of reality and can sense entities who exist in more than one space at a time.
Given the Presence's nature, it is within its ability to be aware of the existence of forces or even realms beyond itself; in this case the Presence is aware of being a construct (a narrative entity) created by a being in a Higher Order or Layer of Reality: the comic writer/creator who breathes life into his creation.
This does not invalidate the Presence, for within the narrative universe of the Sandman Saga, he is indeed the primary force believed to be the creating energy of their particular universe. Given Lucifer's creation of his own reality, he is likely also aware of the Higher Orders of Reality even if his pride will not let him accept it or acknowledge it.
The Second Perspective:
The Presence is the penultimate being in the DC Multiverse, it is both aware of itself and yet recognizes even it can be affected by the only thing greater than itself; the minds of the entirety of reality in agreement could alter reality and itself. The change would not stop the Presence from being itself, only that it would change according to the only force greater than itself.
This is hinted at by Mike Carey when he suggests the answer lies in the story "The Dreams of A Thousand Cats." In this tale, it is revealed that once upon a time, Cats ruled the Universe. Cats larger than men, who chased and feasted upon Humans and rule the world being served by Humans.
- It was only when a Human who grew weary of Humanity being feasted upon by Cats, shouted to his fellows that they could change reality if they Dreamed of a world where Humans ruled the world INSTEAD OF CATS. He would require everyone to believe this was possible and to put the entirety of their being behind it.
- And one day, it was so. They didn't just replace Cats as the rulers of the world, Cats had never been so. Reality had been altered by the will of Humans until the previous Universe had never been.
Is the Dream of Cats lying? Who can say but the consensus was, the Dreaming Cat who sought retribution and change for the loss of her kittens was driven to convince cats everywhere they could have the reality they lost back, if they believed.
But as one old Tom, relayed to a kitten he brought to the storytelling, who could convince a thousand cats to do anything...
If this tale is NOT merely an allegory, then the Presence is implying the Universe is ultimately mallable by a will greater than His. The power of the dreams of a determined group...