The normal phrase is "Uncle Tom" based on the character from Uncle Tom's Cabin. It's used to describe a black person who sells out other black people. Both Uncle Ruckus and Tom from the Boondocks are based on this idea.
But this is a double burn referencing Clarence Thomas, the supreme court judge who is probably the most famous example of an Uncle Tom.
Saying he was the Messiah in and of itself was subversive to established thought.
The Jews at the time thought the Messiah would come in clad in armor, sword in hand, on a white horse, come to slay their enemies.
Instead he rolled up humbly on a donkey talking nonsense like "love each other, treat others kindly"