Basically accusing someone who did something you didn't like of being a racist. The idea is that racists use code words that only other racist understand, like a dog whistle. (So I guess Tim O'Neil must be one.) Some people claim to hear homophobic and transphobic dog whistles from you. Since African-Americans say "rest in peace", your original post could also be considered a dog whistle.

As for the phrase, it's not "pissing and moaning" to point out how ridiculous it is, and if religion isn't sacred, then identity politics shouldn't be either. "Peace|" is supposed to go with "rest", but "rest" and "power" are almost opposites. Further, how is power expressed by the dead? Do they rise from the grave to seek revenge like in EC?