He does not punish Moses and does not give excuses or an explanation. In his patience, God allows Moses to question him and voice his objections.
God is not a dictator who does not tolerate contradiction, but he is a Father full of patience and understanding. And then he repeats his promise. In the continuation (starting in Exodus 6:2), he sends Moses back to work.
We read more often in the Bible that prophets lose hope and complain: Elijah in 1 Kings 19:1–21 and Jeremiah in Jeremiah 20:14–18. God patiently lets them finish what they are saying, and then sends them back to work.
1 But the LORD said to Moses, “Now you shall see what I will do to Pharaoh; for with a strong hand he will send them out, and with a strong hand he will drive them out of his land.”