China is (currently) the highest population country in the world. Unless they want to go full NK and be hermits to the planet, they should responsibly share their information with other countries, so they can get an idea about mutations, severity, spread rate, and so on.

They don't 'have' to share anything. Like a 3rd grader who refuses to share with a classmate. Or they could grow up, and get involved in helping solve Covid before it becomes the next flu.. that we can't get rid of with vaccines due to the sheer numbers of mutations, but is 10 times worse.

I guess to be even more clear, other countries can sure, mind their own business, once China agrees to cut off all incoming and outgoing flights and ships, cut off all trade and travel, and ensure their citizens will 100% not be interacting with the rest of humanity. Since that clearly isn't going to happen, they need to be involved.