The article explains which accounts are being migrated. Of course there's no opt-out, the Chinese government doesn't allow it.
As to the question that the article poses, Apple will surely comply with the law in China. If they have your keys and they are asked to decrypt your data they will do so. They claim they are keeping the keys themselves, same as they do to keep data from other governments. But if you operate in a country and the country asks you to decrypt the data, you're going to do so if you can.
I guess we have to ask ourselves, do we believe Apple is incapable of accessing your account. That's the only way Chinese accounts will be hidden from the Chinese government.
Apple’s hands over only *Chinese based* iCloud accounts to be hosted on Chinese servers in compliance with new regulations. If you are based anywhere else, you’re fine.
The other option is that Apple is completely kicked out of China and Chinese iPhone users are left out in the cold. If you want to operate in China, you need to play ball with their government and their demands. That’s just how it is.