We don't know today how much help they had. e.g. perhaps a live KH-12 feed is handy for guiding a simple missile to a moving ship.
It may not even have been a Ukrainian-made missile. Maybe it was a Harpoon, but they're just saying Ukrainian because NATO doesn't want the public credit.
Russia had 100's of advisors in North Vietnam during the Cold War I don't see why the US shouldn't have support units all around E. Ukraine.
China also actively involved in the war. At the peak of their involvement, 170,000 Chinese troops were present in North Vietnam - [1]
[0] - https://www.rbth.com/history/332396-how-soviets-fought-again...
[1] - https://en.wikipedia.org/wiki/China_in_the_Vietnam_War