I think Kim KNOWS that Trump sees whether he is working on his missile program.
I think someone SAID to him, that Trump wants him to stop work on his missile program, while negotiations are in process.
This seems to be a good sign that Trump got the right message to Kim, and it seems to be a good sign that Kim seems to be negotiating back, in good form.
I really do not know whether anything will come of this, but this seems to be a good first sign.