I think you are confusing what's before the throttle body and what's after it.
What you have said about long inlet runners for low revs and short at high revs is correct. This applies after the throttle body and takes place inside the cast inlet manifold. At higher revs butterfly valves open and air is able to take the shorter (more direct) path to the valves rather than the longer up-and-over path that it is forced to take at lower revs. This is controlled by the ECU which also adjusts ignition timing.
The concept can be seen in the diagram below.
The plastic item that I believe you are referring to runs from the airbox to the inlet manifold mounted throttle body. I think the EF design was to limit induction noise and resonance. It was probably dropped to a simpler design from EL due to cost or manufacturing reasons.