I'm sure there's no case cite, because noone has been stupid enough to argue this before. But the government's point in their stay motion isn't that the 100 documents (or whatever the number is) are classified. It's that there were hundreds (thousands?) of pages taken from mar a lago, some of which the executive before Biden occupied that office stamped as "classified." Trump took documents with this stamp, which we know by virtue of the stamp belong to the executive branch, and which the current executive wants back, and which the current executive, the owner of the documents, says have national security implications. And not that I'm giving Trump's idiotic declassification argument any weight, but it wouldn't matter at all if it were true. These are still documents belonging to the executive. And we know they are because the executive branch put markings on them, marking them as "classified." And the marking signifies that they contain national security information. And now the executive wants the documents it marked back.
The documents don't belong to trump. He isn't king, despite his and Cannon's delusions.