He's a very close second, but the most powerful person in the world is the POTUS.
There is a massive difference between the most powerful position and the most powerful person. You can credibly argue POTUS is the most powerful position, but no one can credibly argue Biden is more powerful than Xi.
Biden's indirectly responsible for a big chunk of Russia's setbacks in Ukraine, helping coordinate a broad response that, from the start, has denied Russia a clear-cut victory.
He's got an understated style. If you think there's no credible argument Xi is less powerful than Biden, that's a feature, not a bug.