Re: Your objection is irrelevant
"If this were a “pool of hired labour” problem, pay would be equal within each bracket, but there just wouldn’t be very many minority / female engineers."
You forgot the scenario of "pressure to hire lots of (certain) minorities and female".
If the pool of talent is smaller, but you *have* to recruit the same number of people (in this case, proportionately), you get a weaker team. It's the same reason that small schools play sports against other small schools, not large schools.