Earlier quoted context omitted.
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The thing about whataboutism is that it was correct . It was only a logical fallacy in the eyes of US jingoists who didn't want to admit that, actually, the US had tons of human rights violations and the US vs. USSR conflict was not a conflict of good vs. evil. When both A and B do a thing X, there are two potential logically sound conclusions: either that X is bad and both A and B are in the wrong, or that it's not…
The idea actually is a valid concept in justice, but only within the realms you posted: If person A does something illegal and then tries to justify the behaviour X with "But everyone does this!", then this is not a valid argument, because either X is illegal or not.
Whataboutism as invented by the economist, though, is just rhetoric nonsense. The USSR was perfectly right in highlighting the human rights atrocities of the US in response to accusations of the US against them. That doesn't make the accusations go away, but it correctly exposes the bigotry.