It's right there in Matthew 15:
22 A Canaanite woman from that vicinity came to him, crying out, “Lord, Son of David, have mercy on me! My daughter is demon-possessed and suffering terribly.” 23 Jesus did not answer a word. So his disciples came to him and urged him, “Send her away, for she keeps crying out after us.” 24 He answered, “I was sent only to the lost sheep of Israel.”
People act as though Jesus was a paragon of virtue, but even according to the Bible, he could be a right bastard sometimes.
They've seen this argument. It gets brought up regularly by people who advocate church-state separation. They don't care, because it doesn't fit their worldview.
I think the standard apologetic is to just ignore the argument for a few days, and then repeat it in the next thread. The other standard one is, I think, to say this was just something they said in order to get the treaty signed, but that they didn't believe.