It's what I've witnessed in person AND what I take from the intended meaning of the concept. Hire/choose/promote/etc certain people over certain other people because the "other" is some stereotype you don't like. It's just rebranded Affirmative Action. It's evil bigotry that happens to make you feel good because of the words they use to describe it.

I've seen DEI used as an excuse to fire and demote people just because they were part of some demographic, to make space for the "disadvantaged" folks. They were good at their jobs, did nothing fireable. They were deemed to be "advantaged" in the same way a random Asian is assumed to be good at math. And now they're struggling to survive. That's the logical conclusion of DEI. Absolutely evil. There's no excuse for it. And that's not a right-wing view either.

What other conception of it is there that doesn't just conveniently gloss over the details of how it works in practice? What am I missing?

Back when DEI was legal, they'd ask questions about groupings like race, which would be factored into whatever school/job decision. Concrete example, Harvard's incoming class is >50% Asian-American now, but it was like 22% when admissions could see race. That was called DEI, right?