That is absolutely true, but it remains to be seen if those attempts will hold up in the long run. There is a big difference between American power being in decline (but still dominant) and the world being multipolar. I would say that currently the derivative is <0 but American power is still vastly greater than any other country.
Of course the Chinese nuclear arsenal is enough in absolute terms to destroy a large segment of the US population (and an even greater share of GDP) but I would not say the same in practice. Contrary to the US and Russia, China has a "no first use" nuclear weapons doctrine. This piece of policy does have material consequences, meaning that the PRC's nuclear arsenal is really just a large stockpile of weapons, not a 24/7/365 array of ICBM bunkers. There is no such thing as a Chinese "red button", but there is an American one. The PRC also possesses no significant SLBM potential, meaning that the US could probably wipe out much of the Chinese land based capability and population centers with minimal losses in return.
You are correct in that there is quite a lot of contention when it comes to the current structure of the international system. While the PRC undoubtedly has a lot of economic heft, the degree to which this actually impacts the "polarity" of the system is unclear. The USSR was not a great power merely because it had a lot of tanks; it was at least seen as a political hegemon that controlled critical territory that allowed it potential world domination. It also had a a foreign policy objective diametrically opposed to the US - leading non-US aligned states to side with it - and parity with the US in the nuclear realm.
The degree to which these are empirically true is itself unclear, but less so that they were norms in the international community (or at least that is what a constructivist scholar would claim). China however does not posses any of these qualities. It does not control critical territory that would expand its influence further (While the south China sea is a vital corridor for world trade, it is not as useful as Eastern Europe for world conquest). China far lags behind the US in nuclear capability, with the US not just a power of ten ahead, but possesing far more advanced delivery systems and a more reliable sea-based force. Most critically it does not have global influence and alliances in the way the US does; this is what I think really differentiates the system from having merely two strong actors and having one dominant superpower. American power comes not from mere GDP or military power, but in the way it has used these capabilities through NATO and foreign investment/trade to project power.
This is not to say that the PRC is not a significant adversary to the US, but I am not convinced that after mentioning that "[t]he historical data show that wide gaps in capabilities, and the possession of very different kinds of power, are common among great powers" one can not declare that "The world is bipolar" from the broad metrics of GDP and military expediture.
If you want to read a different perspective from a realist (although a neoclassical realist instead of what I assume to be Lind's neorealist) I recommend The Myth of Multipolarity.
My intuitive feeling is that this makes too many unscientific assumptions that it does concretely support. The main issue I have is the dismissal of the techniques to kill off humanity. While it is true that it is by definition impossible to try to reason as a superhuman entity it is still the case that a rogue AI would only have a limited set of tools to try and cause a mass extinction. Wouldn't the first step be to cite a counterexample strategy that an AI could use?
I fail to see exactly where this is argued above, loss of control does not make wiping Earth of humanity easier with nukes, but it also moves the goalpost. RAND has deliberately chosen a limited scope that focuses on extinction level techniques, and I don't think that they are trying to offer any comfort on the horrors of nuclear war.
While I am critical of the applicability of AI research on robotics, I agree that RAND's assumption that robots need much more development to be used for spreading pathogens is probably wrong. It is likely that drone technology has already achieved such a level, and that an advanced AI could control a global swarm of them --- either directly or through human actors --- to spread disease together with some sort of supply-chain attack.