Does anyone have the specific Army policy on using slurs and other racist behaviors in the workplace?

Those NCOs were wrong. I'm not sure why they're even EOLs if they're saying that.

AR 600-20, AR 690-12 and AR 690-600.
Download these, ctrl-F the word "offensive" and you'll find everything you need to know.

Unlawful harassment includes, but is not limited to, unwelcome conduct, intimidation, ridicule, insult, offensive comments or jokes, or physical conduct based on race, color, religion, sex (whether or not of a sexual nature), national origin, age (over 40), disability, genetic information, or reprisal when an employee's acceptance or rejection of such conduct explicitly or implicitly forms the basis for a tangible employment action affecting the employee, or the conduct is sufficiently severe or pervasive as to alter the terms, conditions, or privileges of the employee's employment or otherwise create a hostile or abusive work environment.

I was EO back in 2008 and that is not what the Division instructor told us, if you use a term, phrase toward someone even if they don't find it offensive but someone overhears it and finds it offensive you could be in trouble.