whataboutism is simply another word for tu quoque, which, as previously indicated, is a logical fallacy
indeed, the whataboutism itself is intended to kill discussion of the initial topic (in your example, A doing Y, not simply Y), hence why it's a logical fallacy, too
you are absolutely, 100% right here, and whataboutism is the first one, not the second one
in this case, the goal was to stop people from talking about China's actions and instead deflect to someone else's actions
that might explain why Whataboutism, invented by russian propagandists, is explicitly described as a subtype of the tu quoque logical fallacy:
"From a logical and argumentative point of view it is considered a variant of the tu-quoque pattern (Latin 'you too', term for a counter-accusation), which is a subtype of the ad-hominem argument."[0]
as an example, look at whether, after the whataboutism, we're discussing the original topic (Tiktok & its actions) or something/someone else.
see? whataboutism deflection succeeded.
it should be clear now why whataboutism is indeed, as shown above, explicitly defined as a logical fallacy, despite the made-up claim that it isn't one.
indeed, the whataboutism itself is intended to kill discussion of the initial topic (in your example, A doing Y, not simply Y), hence why it's a logical fallacy, too