Wrong.
If the baker had refused to serve the gay couple a product that they made every day that would have been discrimination. The baker did not want to participate in the gay iconography on their wedding cake.
So, if Sarah Sanders had asked the restaurant to cook her a populist chicken with MAGA emblazoned on it they had the right to refuse. BUT if they simply refused to serve her what they were serving everybody else because of her association with Trump that is textbook discrimination.