It's not a pretense. I really don't see how supporting the communist invasion of South Korea, then invading South Korea, then supporting the communist invasion of South Vietnam, then supporting an utterly brutal communist insurgency in Cambodia, then invading North Vietnam, can reasonably be construed as anything other than a particularly odious form of inverventionism.
As far as South Korea is concerned, it was North Korea that invaded, not China. As far as I know, the Chinese took no part in the invasion. The intervention came once it was no longer an internal matter and involved the UN forces driving the North back over the 38th parallel. It seems this is when the Chinese decided to commit troops. Up until then it was considered to be an internal matter of a Civil War.
I'm reading Max Hastings' The Korean War right now, as it happens. Admittedly it is a bit old now and may have been surpassed by a lot of recent scholarship, but he cites Khruschev's memoirs, admittedly another dubious source, saying that Mao Tse-tung were happy enough for North Korea to invade South Korea as they considered the case of the two Koreas to be an internal matter. Thinking of the two Koreas as one country, with an arbitrary dividing line, was quite normal back then and not just among the Chinese.
A civil war is usually considered an internal matter. A revolution is an internal matter. Demonstrations and coups are internal matters. etc... Revolutions are internal matters.
You may have a point with the Khmer Rouge. I don't know how much China was instrumental in intervening on behalf of the Khmer Rouge to help them take power. If the Khmer Rouge were directed or funded by the Chinese for the purpose of overthrowing the government then I would agree that it amounts to interference in the internal affairs of the country. On the other hand, if it was simply recognizing a de facto government then no.
As for Vietnam, we have already discussed this. It is not interference in an internal matter if it is intervention in a war between countries. Vietnam's invasion of Cambodia was not seen as an internal matter.
This is simpler than you acknowledge.