your mechanic is a moron.
the "chemicals" in coolant are for protection against rust and corrossion. The really oldschool coolants and the caltex version of the GM coolant are very effective de-rusters too - but you cannot find them in Pakistan now.
The modern coolants are HOAT coolants which are excellent in their job.
What you experienced is the blowout of some hole that was blocked by rust or something. In my opinion you should be thankful that it happened there and not on a highway when the vehicle would be doing high rpm and speed.