Chinese merchant ships freely navigate the ocean. China is only "boxed in" by Taiwan in the event of a war, not during peace time, and the most likely cause for such a war is China trying to invade their neighbors, particularly Taiwan. If they don't do that, they have nothing to worry about. Nobody is going to come invade China, they're armed with enough nukes to ruin the planet and have a massive military, they have nothing to fear from external threats.

An independent Taiwan is a threat to their plans for military conquest, nothing more.