What purpose is gained from excluding South Korea and talking with North Korea one-on-one, which has been a North Korean demand for 60 years? How can South Korea take it any other way other than undermining them and a weakening of U.S. support?
We have our own issues with North Korea, however. They have been developing missiles that are obviously not designed for the range of South Korea. We can talk to the enemy--talk plainly to the enemy without any suggestion that we have any right whatsoever to compromise South Korean sovereignty.
Trump does not advocate the Neocon insanity, which assumed an American right to reprogram other nations. He would certainly not be talking to North Korea in that vein.