Presumably for the same reason the Gulf of Mexico doesn't show up despite having six of the ten largest US ports. I'd guess they only tracked the open ocean, but whether that's a design decision or those types of clouds don't form over certain bodies of water I couldn't say.
Either way, much of the traffic in the Pacific comes from or ends at China, I don't think anyone is denying that.
So the change in regulation happened in 2020. How then does this hypothesis explain changes in ocean temperature before then? What is the relative magnitude of the correction vs CO2-only-based models?
It doesn't. This model isn't an alternative to CO2 based atmospheric warming models, it just explains a very recent trend in ocean temperature. This branched off a discussion about recent temperature trends. Everyone asking questions about general long term trends either aren't reading or are starting a different conversation.
GP explicitly referenced this phenomenon in an unqualified manner when addressing ocean warming. If they wanted to be understood the first time, they should have clarified the context.