Yes .... his parents came as legal citizens.
"Once they have their Canadian citizenship, and their income is Canadian, they are no longer subject to U.S. tax." .... not true
The story is not nonsense. We also have friends in Toronto who came up in the 70's when they were in their 20's. They have lived here, worked here, paid their Canadian taxes. Never took a cent in U.S. income or investments. Five years ago they proudly became Canadian citizens but did not renounce their U.S. citizenship. They too have to file for U.S. taxes. They too are not happy.
My friend is also a Canadian citizen .... dual citizenship .... it doesn't matter .... if you hold U.S. citizenship you still have to apply.