(Obligatory disclaimer that I'm commenting from the POV of a Black person)

Honestly, it's not even just about calling out racism, sometimes it's actually about pointing out that yes, this thing you are saying is racist. I know someone will think I'm naive for saying this, but, while everyone knows racism exists, and generally what it is, there are sometimes when I have to point out specific things that are indeed racist that come even from well-meaning person, like a close friend or even my white SO.

(Another disclaimer: I'm not referring to the specific case, just thought I'd add my 2 centavos to the discussion)

tl;dr: pointing out racist comments and behaviors can be a service to a person who was lived so immersed in their own world that while they know racism is a bad thing and generally know what it is, they still might need to recallibrate, and having the specific behaviors pointed out is good even for allies.