MUT Device on GTi

Yesterday took the GTi to a Service Centre. Checked with the MUT Device.
The Engineer said everything, ok, BTU, Temperature, ECU only the idling
speed was too low.

Then I asked him, how bout the fuel/air mixture setting, he said that it
shouldn't be tampered with, caused it is fully controlled by the ECU.

But I heard from friends that it can be adjusted to your liking ?

A bit confused right now.

You should never alter the ECU maps unless you know what you're doing and
the PESC mechanic definitely doesn't...