It would also be interesting to see if Johnson ever claimed to have not sent troops to Cambodia. We might as well cover the bases now.
CBS Evening News for Friday, Nov 22, 1968
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Headline: Vietnam / Peace Talks / Action
Abstract: (Studio) North Vietnam blasts United States for South Vietnam's refusal to take point in peace talks.
REPORTER: Walter Cronkite
(Paris, France) North Vietnam holds news conf.; denounces US. North Vietnam spokesperson says secret talks between North Vietnam-US over. Willing to meet with United States without South Vietnam. Castigates United States for intrusions in Laos and Cambodia and for hitting villages in North point of DMZ.
REPORTER: Peter Kalischer
(Studio) State Department denies United States bombs places within DMZ. Says United States takes serious view of enemy violations of DMZ. 8,000 allies fight Communists near Da Nang. 87 North Vietnam and 10 Americans killed.
REPORTER: Walter Cronkite
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Based on the above, it's safe to say that we denied "intrusions into Laos and Cambodia," even though the report never says so (and it would have been typical of reporters at the time not to mention such a denial to make us look bad).
So this does indirectly show that the US was denying Cambodian involvement in late 1968 when Johnson was Prez and Nixon was president-elect.
That doesn't change the fact that Nixon took office later, and hasn't established when the NIXON ADMINISTRATION denied the existence of troops in Cambodia.