Kim knows that  assurances from Tillerson that the US is not interested in regime change means
nothing.  The US has a long history of helping directly or indirectly in regime changes when it suited its national or its global corporate ruling masters interests.  Recent examples of a double cross are Saddam Hussein and Muammar Gaddafi and it remains to be seen what happens with Basher al Assad who the CIA allied with the moslem jihadists have been attempting to eliminate so Syria could be carved up.
Kim has every reason to  not trust the US and knows from history that the possession of nuclear weapons and the means to deliver them is the only true guarantee for his  survival. This does not
prevent him being overthrown from within but given his iron control over the ruling apparatus
and his willingness to be totally ruthless, it is not likely for the time being.