It's credible, but it's not the explanation.

The reason this is happening is that DJT made it clear that he would starve NK until they abandoned their nukes. Then he made it clear to China that the US would hit China's pocket book real hard if they did not cooperate. Then China made it clear to Kim that the jig is up. Kim doesn't want to hang from a lamp post, and neither do his henchmen. So Kim sued for peace.

Journalists, talking heads, etc don't know why something is happening. Only the insiders ( trump/pompeo, moon, kim, etc ) know and they certainly aren't telling the press about it. The noise in the media is just tabloid nonsense to sell ads.

Surely it'd be much easier for them to open a new test site than to finally open denuclearization talks?

I think this is only a contributing factor to their decision, at best.