If the officer had just said "black gangs" or "Mexican gangs" or something like that, then maybe the author read too much into it. I thought it was something exactly along those lines, not necessarily one of those two groups, but something along those lines given that 'mexican' and 'black' are common descriptions for ethnic groups in the US.
by writing "[racial epithet]" rather than "[ethnic group]"
What do you think 'ethnic group' means?
An ethnicity is not an epithet, there's really no connection between the two at all, other than an epithet is a mean name people might call people. And all people happen to be of one ethnicity or another.
communicated to me that the police officer was racist.
Given the weight that some people give to such a charge in today's society, you're really doing no one any one favors by being so brash and cavalier in handing it out, given the paucity of evidence. Even OP should be very cautious with such a label given such a brief interaction.
I think we agree that that shows the author thought the officer was racist
No we don't, I think you may be confused about what an ethnic group is, or the meaning of the term. There is nothing racist about using the proper name for a group of people.
It seems to me as though you're looking in every nook and cranny to find people doing this!
I really should get better at self-editing, I notice how people will commonly grab onto to one little minor supporting snippet of a post while ignoring the real substance of what's been written.