How is that a deflection? It's a direct refutation of your central point.
When a person moves in and takes a job at a grocery store, they aren't just a customer for the grocery store. They are a customer for clothing stores, movie theaters, appliance stores, barbers, restaurants, mechanics, etc.
To simplify things to "basic supply and demand" levels: if you have 20 businesses in town which each staff at a 1:20 ratio, 20 migrants moving into town and shopping at those businesses, increases supply exactly in proportion to the increase in demand.
It's reductive, but then "basic supply and demand" is itself extremely reductive. It's a drastic simplification to illustrate basic concepts, like frictionless surfaces and spherical cows in Physics 101. "Basic supply and demand" is not how economies actually work, it's a simplified model for Econ 101.