China is not a signatory of the UNCLOS. See the south china see debacle for an easy answer as to why.

To my knowledge, China is a signatory to UNCLOS, but has disputes around it's "islands" in the South China Sea and their relation to the EEZ. I acknowledge that China's relationship to UNCLOS, as a minimum, is complicated and rapidly evolving, but I dispute that they do not have a right to transit passage. Or to be more specific, I would put forward that they would have a plausible argument to claim transit passage.

The United States has not ratified UNCLOS, and regularly claims the right of Transit Passage. In fact, this fact is one of the reasons why Iran claims that the United States cannot enter into Iranian TTW while making a Strait of Hormuz transit - because the US has not ratified UNCLOS, their claim is that the US cannot claim transit passage. For the United States (or any Western Nation) to make the claim that China cannot claim Transit Passage would lend weight to Iran's argument, which you can imagine, they would not want to do.

I do not want to make any assumptions around your specific views on this matter - you may hold the opinion that China could not claim transit passage, however I wanted to interject some perspective that:

1. That may not be universally agreed upon 2. Specifically, the United States and it's allies may not make that argument because it would put them in a negative position for other international disputes.

PRC is a signatory to UNCLOS.

PRC actions in SCS is not legally against UNCLOS. Reality is, everything PRC does currently in SCS is legally grey area until UNCLOS/ITLOS/UN actually rule against it in official capacity. PCA ruling is essentially a term paper on official looking letterhead drafted by anti PRC lawfare, pretending to be international law. It's one (strong) legal interpretation, but it's not settled case under UNCLOS - UN (which UNCLOS law operates under) does not recognize or comment on PCA ruling.