Myth indeed. What you are saying is wrong.
They can sell every single miner they build, at a massive profit, they don't have to care about making slower miners.
Please at least say "your guess" when you make such a statement, since fact does not match what you've claimed.
And yet somehow, Braiins and what not are able to re-tune tons of lower spec miners to much higher rates reliably.
The evidence does not support your claim of fact.