On one hand, China probably expected this to come anyhow, and China surely understood the true purpose of this SSBN's presence in the location.
On the other hand, should the US launch nuclear strikes against China from SSBN(s) based in South Korea, China should consider South Korea as a complicit in the American nuclear attack against China - On the basis that South Korea is responsible for the docking and servicing of the SSBN and their crew onboard.
Therefore, even if South Korea does not possess any nuclear weapons of their own, South Korea should be seen as a legitimate nuclear retaliation target by China. This means that once China detects American nuclear-tipped missiles coming towards them from American SSBN that has docked in South Korea before the war begins and/or before the strikes begin, China is fully justified to launch her own nuclear attacks against not just the US, but also South Korea.
Of course, this clause should never be limited to SSBNs only. Strategic bombers, fighters and land-based TELs that are capable of carrying and launching nuclear bombs and/or nuclear-tipped missiles too should be included in the clause.
I believe that sometime into the future, Beijing has to deliver a stern warning to their counterparts in Seoul - privately. The message is this - Either the South Koreans can stop the Americans from stationing their nukes that are directly pointed at their neighbour next door (in exchange for Beijing restricting Pyongyang's actions north of the 38th parallel), or they can witness their cities getting flattened with their very own eyes.
Meanwhile, it would be better for China to make this warning public only when sh1t is about to hit the fan, e.g. full-scale war between China and the US is in the horizon, or that visible signs are pointing towards potential nuclear first strikes against China are being actively considered by Washington DC.
Of course (again), this clause should also not be limited to South Korea. Any country(ies) in the Indo-Pacific region whose actions can be interpreted as a complicit of this should also be targetted as a justified nuclear retaliation move by Beijing.