The wonderful husband insists that the last time (years ago) he flew from Canada to the US via SEA, Canadian flights were seperated from the general immigration crowd. I'm thinking he means that Canadian citizens can use the US passports lane. I'm arguing that all flights from Canada are nowadays treated the same way as any other international departure and you will have to stand in the same are as the Hong Kong flight that landed at the same time.

Who's right?

Not sure I follow, as you mix departure and land in the same sentence.

Passengers flying from Canada to the US clear immigration before departure in Canada. Hence, upon arrival in the US one simply deplanes as if they were on a domestic US flight.

We're flying out of Victoria. They do not have pre-clearance there, so we have to clear US immigration when we land in Seattle. My husband says that flights arriving from Canada gets segregated from the other international arrivals, and I'm saying not.

Flights from YYJ go to (typically) the south satellite at SEA where there's direct access to the US immigration and customs zone. Long haul international flights (and IIRC flights from Kelowna, which also doesn't have a pre-clearance facility) also land at the S gates. I believe sometimes YYJ flights will go to another gate, but pax will be bused to the I&C hall.

All the flights from YVR, YYC, YEG and YYZ pre-clear, provided they depart Canada while the US immigration/customs desks are open at those respective airports.

I went from Victoria through SEA last year and I had to go through immigration and customs the halls were packed and it took almost an hour. They did have a immigration line for US citizens but the line was long so it took time.