Yes, it's true that domestic flights exempt pax from having to reclear security. But I don't understand why some people who arrive airside (from iternational destinations) require reclearance while some (domestic) don't. It seems Britain does not regard non-British security clearance as valid, cf eg Germany where I can be accepted EDI-FRA-ORD without reclearance in FRA.

Actually if you fly LAX-SFO-LHR they make you reclear in SFO. But that is down to the physical setup of the terminal. No point arguing with the TSA.