I've read all of Zeihan's books, and you may be emphasizing different points than those he focused on.
>US has more navigable river
You may be right. You quoted the poster, who said they quoted Zeihan. Are both countries blessed in this regard? I guess this affects transport, like you mentioned, and farming. I don't remember these particular details from the book, I'm sure he's written hundreds of words on the rivers in these two countries. Does China have enough fresh water, and farmland they can irrigate? If so, then this might not be as big a deal as other factors for these two countries.
>has great ports on both oceans.
You mentioned how many large ports China has. I assume that is because they have been exporting so many manufactured products to so much of the world over the last couple decades. Zeihan seems to make a case that this might not be true for long, due to demographics (there will be more Indian labor, Mexican labor local to the U.S., Columbians and Vietnamese working even cheaper, better Japanese robots, etc) and also due to changing circumstances of international shipping (if the U.S. stops securing shipping around Eurasia where does that leave China? Can they reach their trade partners? Import resources? Do they have capable enemies that they will have to deal with?)
>is self-sufficient in energy
Zeihan made a big deal out of the U.S. fracking developments over the last ten years. He didn't mention this is the context of war with China. He said that because of the end of the cold war and recent advances in fracking it is not longer in the interest of the U.S. to police the worlds oceans, secure the Middle East, etc. He said that would leave other countries, like China, India, Japan, countries in Europe, etc. in a position where they need to secure their own oil supplies, oil still being a very important energy source, and trade routes. All these other countries have enemies and friends. He's saying that the U.S. can withdraw as much as it wants, for the most part.
>has horrible demographics
Zeihan has talked about demographics a lot. Not just China but many other countries will be dealing with the consequences of lower birthrates - an aging and shrinking population. As countries become richer their birth rates go down, eventually below replacement rates. There won't be as much young labor, nor middle aged investors, and there will be more elderly people to take care of. How dependent are our systems on growth? What happens in various places when population and gdp are shrinking? He says that China will have it's hands full over the next ten years dealing with this, but that the U.S. has a large generation of 30-somethings that postpones our "decade of reckoning". Different countries are in different demographic situations. Japan is a rich country that is currently dealing with this situation. Russia is dealing with this also, but not doing so well.
>US has kept shipping lanes open, and China relies on that to this day
You mention that China now has a deep water navy. Based on my reading of Zeihan, I assume they will have a chance to put it to use eventually. Depending on how things go they may need to secure oil shipments from the Middle East, and they could be in competition with Japan and India for that and other resources.
>fairly hostile island nations with lots of antiship missiles
I just don't recall Zeihan talking much about the U.S. going to war with China, certainly not for world domination, like the cold war with Russia. He seems to be saying that there's no reason for the U.S. to start a confrontation with China, that long term it makes sense for the U.S. to continue the trend of withdrawing. I assume he's talked about a possible conflict around Taiwan, but it wasn't central to the trends he focuses on. I know that shortly after sanctions were imposed on Russia, after the Ukraine invasion, he talked about how similar sanctions might affect China, and how that may affect China's thinking on Taiwan in the near future.
He said the U.S. was most likely to approach future conflicts like the recent conflict in Ukraine, using sanctions, weapons, and intel to support allies when it is in our interest, but not sending troops.
>China has become a great economic power
I don't think anyone would deny that. The last few decades in China have been amazing! Unprecedented in history.
I agree with you that politics can be relevant. This is something Zeihan has talked a bit about. He's mentioned Argentina as being a country with a lot of potential, if they could get their politics together. He's talked a bit about internal China and U.S. politics. But he talked much more about the upcoming challenges, different for different countries, due to a changing world order, demographics, and geography. He said there were going to be a lot of failing countries in the next decade or two, more places like Sri Lanka. The world will not be as stable as it has been.
I wasn't sure why you talked so much about a direct confrontation between China and the U.S. If anything Zeihan seems to be predicting that China has demographically and economically peaked, and that they are likely to implode in the next ten years.
I consider Zeihan to have an interesting perspective on all this. I take him as pessimistic food for thought.