Simply not true. It is very easy to work as a Canadian in the US. This is exactly why we have the TN visa.

Because of NAFTA, the US and Canada share a ton of workers.

The thread is about remote workers, not immigrants. He's talking about a Canadian who continues to work remotely in Canada, not a Canadian who's moved to work in the United States.

It makes no difference if he is a US resident or not.

Of course it does. If you pay someone when they reside in the United States, you don't have to deal with the Canada Revenue Agency or any of the provincial and local authorities at all. Instead, you deal with the same federal, state, and local authorities in the United States that you're already dealing with. Navigating bureaucracy is hard, and navigating additional unfamiliar bureaucracies is harder.

Not only was he not a US resident, he was actually working in Candada, so whatever labor laws and tax rules normally apply in Canada applied to him.