According to here the US Customs service at Toronto Pearson YYZ only opens at 6 am. Nevertheless, Air Canada has flights at 6+ am to New York. In that case, how could you clear the border and board the flight?
Just 7 days ago from the time of posting this @TorontoPearson tweeted:
Customs open at 3:30 am and counters are open 3 hours prior to scheduled flight times. Hope this helps. Regards.
Therefore, the hours you listed are out of date.