Unless the Chinese are silly enough to spend billions converting a portable launch unit into a submersible launch unit, I'm not sure what point there is to bring it up. Ballistic missiles being used against slow moving targets has been proposed, explored, experimented with, and ultimately discarded. If you have a overly cautious and easily spooked enemy, launching any ballistic missile for any reason is going to elicit a strong and likely fatal response. And if you've managed to come up with a method of differentiating between a conventional or nuclear payload, I'm sure a whole lot of countries in the world would be very happy if you shared that information immediately.
An anti-carrier ballistic missiles is money well wasted. That said, the real danger is in China selling these portable launchers to other nations who'd be stupid enough to use them. So long as they're in the hands of a nuclear power, they're too dangerous to use.
The DF-21D anti carrier missile was based on the Persing II missile. They managed to turn it to a game changer in the China Sea.Anyone who manages to reverse engineer a former missile that was in the US nuclear inventory is extremely dangerous.