North Korea has never recognized the Seoul government. Their demand since the war has been for the U.S. to deal with them alone and not in partnership with South Korea. Trump wants to give into their demand. There is no way that South Korea can take it other than abandonment, and when added to Trump's recent statements that he will insist that South Korea pay us for helping defend them then they're probably right.
It would be interesting to see if suddenly in that case the ChiComs suddenly switch their allegiance to South Korea. They've been cozying up in recent years, and the one think China and South Korea do not want is millions of Nork refugees pouring over their borders.
Talking with them is not dealing with them in any way that would undermine our ally. South Korea, as Western European nations, should certainly pay for our help. But note, Trump also recognizes that Japan & South Korea might be well advised to develop nuclear weapons to protect themselves from North Korea. (Frankly, I would strongly suspect that they have already done so, quietly.)