Whataboutism is when you are trying to show a double standard because person A did a thing that you were upset about and person B did the same thing and you don't care. Asking why you care about a person doing a thing and didn't care when a different person did something different is not whataboutism.
I agree with the GP on the definition of whataboutism, and think that you have described something with a much older term: "pointing out the hypocrisy".
They both share in common that rather than continuing to talk about just one thing, you are now talking about (at least) two.
But whataboutism is a diversion tactic that tries to shift the attention from behavior/event A to behavior/event B; pointing out the hypocrisy notes the similarities between behaviors/events A and B and contrasts the response.
Both can be deployed in similar situations, but the motivations for choosing one over the other are substantially different.
