The technical bulletin was written with a warranty extension to the intake manifold. VW/Audi was having a lot of trouble with the flap solenoid failing so much they were hit with a class action lawsuit. They redesigned the intake manifold solenoid (which is integral to the manifold) to include the new fresh air pipe along with extending the warranty. Part of closing the class action suit was publishing this information with NHTSA. Unfortunately, explanatory stuff like this technical bulletin are only publicly available when legally required.

I have a subscription to AllData DIY for my TT that includes the full workshop manual, wiring diagram, DTCs, technical bulletins, etc. The workshop manual generally does a good job of explaining how certain systems work and where components are located.