Ummm, I'm a bit fuzzy on my history. Would have to look up which president made fairly public pronouncments that "troops were not in Cambodia." I don't recall Johnson making that sort of statement ... but maybe he did. It's a good history "look up," FWIW.
I say "FWIW," because it is clear that Kerry is a phony, that he makes things up to suit the moment, and he is willing to wildly inflate facts, even about himself, in order to obtain personal gain.
No one said we were in Cambodia in 1968 as far as I know. We started secretly bombing Cambodia in March 1969. We didn't invade Cambodia until April 1970.