It's a garbage article. Instead of saying what it really is - you go through US immigration and customs in Britain, then the flights are treated as domestic flights when they hit the US.
It would certainly require a dedicated terminal for US flights, and I don't think that's possible at most airports. There are a huge number of flights to the US from there, on a lot of different airlines. An airline like BA would have a triple system of terminals/gates in use at LHR. One for US flights, one for domestic/CTA flights, and one for all non-US international flights.
Preclearance makes sense at the bigger Canadian airports, where a huge percentage of traffic is going to the US and on US carriers.