> "My opponents are lying racists" would be the ad hominem fallacy even if it was true.

No, if it were true, standing on its own, it would be an accurate statement of fact. It is only be the ad hominem fallacy if it forms part of an argument with this logical structure:

1. My opponents argue X, but

2. My opponents are lying racists, therefore

3. X is false.

That is the structure of the argument. X is that DEI is a bad policy.