Having travelled to both Canada and the US for a dodgy-looking (but in both cases totally legitimate) 3 month visit, I can tell you the Canadians are very much stricter than the American border guards.
This has not been my experience, but both are much stricter to those who are not their citizens, since they're pretty much charged to be. If you're an American entering Canada, expect to get asked why, and vice versa. It seemed like the OP was trying to enter on a non-work-authorized visa waiver, and that's just not kosher to USCBP. I say this as a US immigrant from Canada.
Always make sure you have your ducks in a row at the border. I still find it hard to believe that someone approved to YC would not know that. If you're doing anything more than just visiting, you probably need to apply for a visa in advance. Lying to CBP (they call it "misrepresentation") is grounds for a lifetime ban from the US.
