LHR international arrival confusion

In the last episode of , Mark Carroll
said:

Thank you very much to you and others for explaining. Hopefully they
have an easy corrective route for those who followed "flight
connections" not realizing this! (It would be an easy mistake to make
given how, coming to the US and connecting domestically, one gets quite
used to picking up one's bag in flight connections, for port-of-entry
customs reasons.)

The key here is that the port-of-entry to the US was in Toronto, after
that you were US-domestic for the rest of your journey.

Unfortunately that's a complicated enough concept that most airline
officials probably won't try and explain it to your average traveler
since it holds up the check-in line. Sad but true :/

--
We have enough youth, how about a fountain of SMART?