It is, maybe as a second order force but if Google, Facebook etc can pay Engineers those salaries, it's only because of their already well oiled scalability tha

It is, maybe as a second order force but if Google, Facebook etc can pay Engineers those salaries, it's only because of their already well oiled scalability that those Engineers built; if they were selling anything with tiny margins, they could not have a large percentage of their employees have a huge salary. And this is what generates demand, which generates scarcity of talent, which makes both what you say and what I said true. To understand that scalability (/margin per employee) is true, a good counter example would be: if there was a constrain of McDonnalds employees for some reason and that persisted, the company could not afford to pay them $100k salaries because it just doesn't make that much profit per employee (does not scale).