The mechanic might have been a "back room" guy and not totally aware of their inventory. Simple ignorance vs malice.
Sometimes, the factory tires aren't the best tires for your specific use.
If you only ride on smooth pavement, a slightly smaller slick tire would roll easier and make the bike accelerate easier/quicker.
The Specialized Armadillo tires on my Globe were horrible. Their rolling resistance wasn't much better than knobbys.