Your tube is above 8mm and does NOT require the diffusor.
The requirement is stated a little more clearly on the
PRODUCT PAGE:
Important notice:
If used with tubing with an inner diameter below 8 mm (for example plug & cool), for models mps flow 200 and mps flow 400 an extension (art. no. 63035) must be installed into the inlet port of the sensor to harmonize coolant flow. Without extension, the sensor readings are not valid for these configurations!
Edit: and it's the fitting more than the tube.