Whataboutism doesn't even make sense as a rhetorical fallacy in most contexts redditors tend to use it in!

An example of an exchange the accusation of whataboutism would make sense in would be if a Chinese citizen was making a good faith criticism of some policy the Chinese government is enacting. In that case, saying America is doing the same policy wouldn't make sense because that sidesteps the whole point of the criticism: to get China to not do the thing they were doing.

On reddit, most criticisms of China are not made in good faith and are really implicit arguments comparing the virtues of capitalism vs socialism with Chinese characteristics. That is to say, they are trying to find some aspect in which China is worse than western countries. They do not believe in any way their criticism will actually impact China for the better, nor do they care if it does.

This is exactly what is going on in the screenshot above. The redditor is saying the (actually Mexican) building is probably Chinese because the quality is not up to snuff with what they perceive western buildings to be. In that case, it makes perfect sense to use a shitty western building, such as the Grenfell tower, as a counterexample. But nope, can't do that! China exists in a vacuum, any flaws as compared to a theoretical perfect country means the Chinese government is evil. Actually good governments lead countries that are perfect in every regard!