The issue with this logic is that you assume the candidate stream can't be changed/expanded.
An example of a business decision that could be influenced by DEI hiring: rather than doing the usual outreach programs only to the usual universities, you could decide to turn stones that you wouldn't usually turn in order to find people you wouldn't usually find. This has zero impact on the quality of people you hire.
Also, "We only hire the best" is a bullshit catchphrase designed to flatter the ego of fresh grads in order to soften them for comp discussion. What usually happens is that you hire whoever passes an arbitrary bar you've set, and you're content not to have and waste another month in a new interview round before you start your new project.
> Just making sure that your hiring is not actively racist itself is not something I'm arguing against (and I would not call that "DEI hiring").
edit: Unfortunately, it is. e.g. someone created a throwaway account to peddle weird fantasies in answer to this message.