Definitely no ITI at SEA, but the tamper-proof bags from duty-free are fine on connecting flights. TSA security procedures are the same no matter what your destination is and the screeners don't know or care where you're going--you're reading too much into the website

I used to do it all the time, POS-MIA-SDQ without checked luggage. Even with overnight layovers in MIA, my Trinidadian airport booze made it home without incident every time. TSA did occasionally take the bottle(s) out of the bag to scan them.

Where you could run into an issue is if you're connecting again in Canada, but based on your username I suspect that's not the case.