And they agreed in writing to denuclearize.
Either "in writing" matters, or it doesn't. You can't argue that it's binding when the US does it, but not when North Korea does.
I can see how you can be misjudging the success of this summit if you think this actually happened. It didn't.
What North Korea agreed to, in writing, was to "work towards complete denucliarization of the Korean Peninsula".
To those of you new to the diplomatic lingo in general and North Korean negotiations in particular, let me translate:
1. "Agreed to work towards" is not the same as "agreed to do something". In particular, it doesn't specify North Korea doing anything in particular to any particular part of its nuclear program.
2. "Complete denucliarization (of the Korean Peninsula)" doesn't mean what you think it does. Basically, it implies US troops recall and pulling the US nuclear shield from South Korea.
So what you have is a piece of paper essentially stating that if the United States goes home, Korea may start destroying something at their own pace and without a specified mechanism of monitoring and control (not even a broad one). I'm sure the Chinese are ecstatic.
This might seem like conjecture to you, but we've already been there with North Korea, and that's exactly how it panned out.