Your understanding is 100% correct (can't write anything more than that)
If going back the same way, however, you'll clear US immigration in Seattle, then collect and re-check your luggage at the designated counter after customs.
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Therefore, the hours you listed are out of date.