As one who has just moved back to Toronto from NYC after living there for 22 years, I would agree with the general sentiments in the article. On the matter of how many Canadians live in New York, or LA, I think the correct answer is "no one knows". While census data may count some groups, I'm not sure Canadian is a category. And, while the Canadian census seems to be relatively thorough - at least my apartment was always surveyed even when I was not living there, the US exercise is about as meaningless as voting Democrat in Florida. In my 22 years in NYC I was not surveyed even a single time. The Canadian consulates/embassies have registers but I know of no one who ever signs in there, even in risky places which New York is not.
But anecdotally, the number of people I met over the years who were Canadians working there, who were studying there, who had been born in Canada but had became US citizens, who had spent a few years in Canada when growing up, who had emigrated to Canada and then subsequently moved to the US, etc. was enormous. Add to this those who had spent school years in Canada, has worked for a number of years in Canada, etc. and the number becomes even larger. Then add in all those people in New Jersey and the commuter belt of NYC who work/shop/spend time in New York and I am sure you get a prety respectable number, certainly more than LA.
The thing with most Canadians is that they can "pass" for Americans. In fact, like all those south of the Rio Grande, we are Americans - but that's a discussion in semantics. A social security number is not difficult to get and no one seems that anxious to look too closely at people working in the service industry anyway - and this includes what seems like the million young people who have a day job while they pursue artistic/acting careers. It was rare to be in a downtown restaurant or lounge where there wasn't at least one Canadian working.
As TonyV says, it is interesting that a US journal is writing about Toronto as a place and not as an origin for comedy writers and comedians, hockey players and indie bands or as a location for New York movies.