I haven't been following what Trump has and hasn't said/tweeted recently however if Trump made that statement I am wondering if he said it in the context of saying they were not involved from the standpoint of working with his campaign? I think it's been fairly well documented that foreign powers, Russians, Israel, China and others have taken interest and potentially made moves to influence elections in America one way or another to better suit their interest.
Not to be hypocritical either it's pretty well established that the Obama Administration, and I'm sure many others, have tried to influence and affect other Nation's elections for the exact same reason, for our own benefit in hopes to have someone that we feel we could work better with or that is more favorable toward the United States. I'm sure we have had our meddling fingers in many different pies throughout the world so no political party, politicians, or group of people should act so innocent as to act like it doesn't or hasn't happened before or will not again.
It's also fairly well documented, now, that Hillary Clinton's campaign and/or the Democrats paid for, and used, information from the "Trump dossier" that was secured from a former Brittish agent involving help from some Russians. As to who the exact Russians were and whether or not they were employed by the Government has not been revealed, to this date, as far as I know.
As for Kim Jung-un and sis, I have to say they aren't as nieve and dumb as some people tend to indicate they are. Could be though they are adept at listening to advice from China and Russia either of which could have floated the idea and notion for North Korea to seek mutual cooperation and walk in as one Korea with South Korean athletes. It was a savvy and calculated move on their part.