Those laws were themselves a direct consequence of imperialism. Without the empire there would have been no Commonwealth migration to legislate about. Public opinion might explain why the laws were tightened, but it’s irrelevant to the causal chain.

Speaking of causality, Kirkegaard did not perform any kind of causal inference, so his analysis is based on correlation only, not on identified causal effects. He compares group averages and finds correlations with outcomes like employment or fiscal contribution, but that’s descriptive statistics. There are no counterfactuals, no identification strategy, and no attempt to separate selection effects, institutional factors, or assimilation dynamics. In other words, it’s not causal evidence — just patterns that he interprets as if they were.