How does the pressure change in the Air Cycle Machine?
Strictly speaking you're right, the authors used a brief and easy to visualise but slightly inaccurate phrasing. The blue bit is open to the cabin, and at the end of it will have cabin ambient pressure. At the beginning of it the pressure will be slightly higher, but very much lower than before the turbine.

The pipe diametre does not need to change for the pressure to vary. Air streams from high pressure to low pressure, upstream pressure will always be higher than downstream pressure, by an amount to overcome the friction forces.