I do not think that this really means that Russia is making the threat real. Let us be ambiguous in the best style of diplomacy. Yet the arrival of several russian divisions would plausibly get China off its backside to perhaps resolve this whole inconvenience.
Alternatively President Trump could send several Aircraft Carriers into the Sea of Japan to provide close air support for the motorcade of Dear Leader.
Whatever the result i do think that negotiations to end the Korean War need to be reopened. Preferably on the Deck of a USA aircraft Carrier and broadcast globally while been conducted. The mere threat of that much loss of face may actually win concessions.
I also know how to play unreasonable.