I don't quite understand what the conservatives think DEI actually is, because as far as I can tell three women being in leadership positions in the LA fire department is "DEI". It just seems like the new "woke" or "political correctness", and just as vague.
It can range from explicit efforts to actively reach out to candidates from schools with students predominantly from historically-disadvantaged groups, to outright "no hiring of white men this quarter" (which is one of the more egregious documented cases, https://www.theverge.com/2018/3/2/17070624/google-youtube-wi...).
What I think most conservatives think it is: When choosing between a more qualified white cis man and a less qualified "diverse" candidate, some companies will pick the latter specifically due to DEI policies. I am very sure this is actually happening (see also e.g. the above source).
Turns out people don't like being explicitly discriminated against, even if they're from a majority group.