That's cool! I never tried to get around the 20mm and we religiously test every suspicious package if it fits, I cut the 18mm hole in cardboard (to have some reserve).
I talked to post office clerk yesterday, and she told me it did not make any difference if we came 8 in the morning or 4:30 in the afternoon. So that particular case, they were just fast in Stoney Creek sorting facility during night shift.
Yup, lettermail is pretty nice service. Postman will take it to your mailbox, but some postmen will not bring you parcel. I just had a 0.5kg parcel (it was USPS Express flat envelope) and I had to go to pick it up.
It's good CanadaPost allows sending non-documents by lettermail. Some other countries national posts do not allow sending non-documents by lettermail. Perhaps in the future we will see the Light Packet for Canada as well, that's what happened with international non-document lettermail two years ago approx in August/September. Same service as international lettermail used to be, they just bumbped the price to double.