The chain rollers wear at the bearing load surface in each link, this causes excessive clearance in the link bearing & adds up to a longer chain when measured end to end or compared to a newer chain. Each link having play can quickly add up to a loose chain. This would account for using the term "stretched". I spent 25 years working on cars for Dodge, Chevrolet, Honda, Audi & Porsche. I don't think they actually "stretch" until you get them over 600 degrees or put a blower on top of the motor.
To the original poster, your mechanic "Technician" is correct, he has to almost remove it to tell if it all has been replaced, my advise, if the car is nice & you plan on keeping it & taking trip etc, get a new t-belt & w-pump & be done with it, otherwise you are headed for a surprise valve crashing party out in the middle of no where and I bet you will be short on beer for it. good luck