The classical miracle argument relies on the idea that God's extraordinary confirmation of a claimant is not to be taken as an endorsement of a false divine mission. That theological-rational premise gives the miracle its evidential force.
A ruler publicly authenticating an envoy through a recognized sign provides an analogy for confirmation, not an exact comparison between divine action and human government.
The conclusion is warranted only when the prophetic claim, the reality of the sign and its confirming relationship have been established. Removing any premise weakens the inference.