"Immigration enthusiasts are so hysterical"

This quote by Peter Brimelow suggests that those who are in favor of immigration are overly emotional and irrational in their views. He implies that they are not looking at the issue objectively and are instead driven by their own feelings and passions. He may be implying that they are not considering the potential consequences of immigration, such as the economic and social impacts it can have on a country. He may also be suggesting that they are not looking at the issue from a legal perspective, and are instead relying on their own personal opinions. Ultimately, Brimelow is implying that immigration enthusiasts are not looking at the issue in a balanced and logical way.