China is the largest holder but she doesn't hold a lot. Over 80% of US T-bills are held by various sovereign states, corporations and banks. To put things in perspective, a few smaller states holds as much T-bills as China (combined) eg. Luxembourg, Cayman Islands, Ireland, Belgium. And Banks and Corporations hold much more than China.
In my humble opinion, China should not release the US T-bills in her coffers, if she wants to devalue yuan as a weapon against US in trade war. If China wants to weaponize yuan, it must be backed by a huge quantity of T-bills and gold, or else yuan will risk a meltdown like Indonesian rupiah and Thai baht