Surely, all the border guard will ask is "what is the purpose of your trip to Canada", they'll respond I'm in transit and that's the end of the conversation. Perhaps the border agent could check the documents, boarding passes, but it's not a given. Before the Newark-St. John's flight was cancelled, I would transit KEWR fairly frequently from Europe, not once did US officials check my next boarding pass. The person in question was legally in Canada (6 months without visas/permits) and as noted above would have had to fill out an eTA whatever.

Lets say the agent did check the documents, and found he'd made a monumental error. The agent would still let him in.

Incidentally, my mum who visits 3-4 times a year thinks the eTA has made entrance into Canada easier, says the agents ask fewer questions and process quicker now than before.

Regarding this story, I was a little bemused why this person decided to tell people. I'd be ashamed of such stupidity - I'd have gone home and kept a low profile. But looking at some of the other news outlets and small snippets of extra information, it appears he's set up a crowd-funding page to fund a trip to Sydney NSW, because he blew all that money on going to Canada.

Which is also pretty stupid - he's publicized his mistake to try and get sympathy and money. But most news reports aren't interested in the crowd-funding anyway, but the mistake. So it hasn't worked.