Didn't Japan and S. Korea have a diplomatic meltdown while Trump was in office? I remember their hotlines being disconnected or something. Anyway - this meeting symbolizes a return to normalcy between the 3 countries.

At this point it seems clear to me that any support China gives Russia is an attempt to avoid a full meltdown of the Russian state and the possible repercussions of having a failed state or a Russia in a civil war on their north border. Beyond that I would be surprised if they see Russia as a useful ally in any way now that their weaknesses have been exposed to the world.

If they help Russia it plays into American hands, by pushing Europe closer to alignment with US allies. Overall it would be a big loss for China to intervene because of this, which is why I think they are staying put of it. They don't realy have good moves to make here, only less unpleasant one. Thye either let one of their only strong allies bleed or combine collective west in an anti Chinese expansion dogpile.

It wouldn't be an easy decision. I think for now China and Russia would hold-on on ratifying a full Alliance as it would have a some immediate draw-backs for both of them. Japan and South Korea were always pro-US and it was always assumed that those three would be on the same side in case of a war in Asia. China might make offers of it's on to various nations in Asia in response, but they will probably delay formalization of a "no limits" partnership with Russia for now.

I’m interested to see if this move pushes China to become more aggressive in the SCS or to formalize their “no limits” partnership with Russia given their recent defense ministers meetings

Sounds like it stopped short of mutual militery commitment. If that is the case I'd really call it maintenance, there doesn't seem to be fundamental upgrade of relations.

The rise of China naturally forces them together. It's amazing how long the cultural animosity has lasted, in any case.

On the military front, the US already have security pacts with Japan and South Korea, so I'm not sure how significant this is. From the linked article (and other articles on the same story), it seems the only noteworthy development is the setting up of a new direct hotline between the three, which is not all that exciting. On the economics front, nothing concrete has been actually been announced, but the three are already in very good trading relations so I'm not sure what can come on top of that.

Why didn't this happen during the cold war? The PRC was far weaker during the Mao years, so any response from them would be impotent.

This seems to be why China won't get involved in Russia, if I had to guess. If they do they will without a doubt pull Europe closer to our eastern allies and make a unified global West in nato a real possibility, which is the end goal of this policy, at least I hope so.