You aren't worried about the certificate itself being forged, all it contains is details on the block that the document was signed on and the hash of the docume

You aren't worried about the certificate itself being forged, all it contains is details on the block that the document was signed on and the hash of the document, you can't forge this since you still have to make a SHA256 hash of the document and verify it against the blockchain. The whole point of this transaction is the blockchain itself records the time that a specific block was mined, if the hash of the document was in that block then that means the document did in fact exist at the time.