Anything you do to act against or weaken a given state tends to benefit that state's rivals. Unless you have some switch you can flip to overthrow every government at once, then either you refrain from meaningfully opposing your government or you make peace with the possibility that another government may benefit from your actions.

There are cases where leftists should refrain from opposing their government, for example, during WWII, it wasn't the best time to strike or revolt if you lived in the US. On the other hand, in WWI every side was relatively the same, so there was no need to put class war on hold, it doesn't matter if German capitalists or French capitalists came out on top, so both German and French workers would have been justified to strike and disrupt and revolt, even though French workers doing so would benefit German capitalists and vise versa.

Therefore, this "oppose all states" line is reductive. If someone paints the PRC as being so bad that they're a threat on par with the Nazis, so we should put class war in the US on hold in order to stop them, then we need to evaluate whether or not that's actually true. Which means first making the assessment, and then, if we assess that it's not that serious of a threat, that means "defending" it (at least to an extent), it means saying things that will get you labelled as a tankie. If, on the other hand, you don't do that, if you concede even the most extreme and far-fetched claims about them, then either you must also concede that we should drop everything to fight them, or you must adopt the unreasonable stance of weakening a state that stands between you and something equivalent to the Nazis.

Any actual opposition to a government (as opposed to opposition merely in principle) means at least being ok with that government's geopolitical rivals benefitting from the internal conflict. Actual opposition will almost always mean getting accused of supporting enemy states (whether you do or not), or at least of not taking their threat seriously enough. It's necessary to have an answer for why you're not rallying around the flag to fight off that supposedly greater evil.