A very important thing to note is that he believes the U.S is gone as the sole superpower. What he really means by collapse is about the same fate that endured the British Empire.
Even assuming the US doesn't have anything called a "collapse", it's obvious to anybody with eyes that there are a number of countries that will inevitably grow to superpower status over time. This is normal, and arguably good, since the alternative ("everybody stays poor") is probably bad. Singulatarians would argue that tech is moving such that people, or at least small collections of people (on the order of tens of people), may become "superpowers" by modern standards.
Even the most optimistic predictions of continued US success don't have it coming at the expense of other success, and the economic phenomenon that less developed countries have much more low-hanging fruit, allowing them to grow to modern standards much faster than the leaders, will not stop anytime soon.
Seems to me the question of whether the US will "collapse" is distinct from the question of whether it will be the "sole superpower" across any timespan past 10 or 20 years. (Slightly later edit: And the collapse of the US won't make China a superpower, it will mean there simply isn't one for a while. Superpower is ill-defined, but it definitely has something to do with being able to project force around the world, and China can't do that yet, except with nukes, and if that makes a superpower, than they are already there, along with numerous others.)
In the end I decided that you mean: People call out the bullshit on HN.