That’d be like saying “The people of the Confederacy did not support the Union invasion”, with “the people” only referencing the white citizens of the Confederacy and not the millions of black slaves, Native Americans, and white abolitionists.

Of course the “people of Tibet” would not be rejoicing at the PLA operation; as those people were part of the 5% of the population that owned the other 95% as slaves. I’m sure that the 95% were distraught they were no longer enslaved against their will.

Also Haiti didn’t have slavery because they themselves wanted it. It was imported on a mass scale from a colonial power. Tibet had slavery because it was baked into their religion and culture with a vile caste system and violent repression.

A country that is built upon 95% of their “citizens” being chattel slaves does not have a right to exist and be tolerated by countries around it, and such a state genuinely should be invaded if the aim of the invasion is to sweep aside a vile system that is keeping millions of people as nothing more then livestock.

I want to correct a few misconceptions that are evident in your comment:

I'm not convinced the Tibetan people supported the invasion.

It wasn't an invasion, it was a liberation, and a peaceful one at that until reactionary forces, fearing the loss of their privileges if the serfs should be liberated, started a brutal armed insurrection.

The vast majority of the people of Tibet at that time were serfs and lived in miserable, inhumane slave-like conditions. Are you arguing that slaves would prefer to continue to be enslaved?

justified by "slavery existed in Tibet"

That was indeed not the primary reason why the PLA first entered Tibet. It was rather to preserve the territorial integrity and safeguard the sovereignty of China, of which Tibet was and is recognized as an integral part.

by that logic any country with slavery [...] deserves to be invaded

Tibet was not and is not a country. For a period of a few decades during which China was in chaos and turmoil following the fall of the Qing dynasty, the local government of Tibet had merely ceased to answer to the central government of China, but the region never formally declared Independence and never ceased to legally be a part of China.

Pro-independence forces fomented and backed by western imperialists who had already once invaded Tibet were however attempting to break Tibet away from China just like they are trying to do with Taiwan today. This created an urgent necessity for the PLA to intervene to protect Tibet and secure its borders.

I don't think the motivation was to free the slaves.

Whether or not this was the primary motivation, this was still one of the main goals that the CPC openly declared needed to be accomplished sooner or later, as it was evident that the system of feudal-theocratic serfdom was halting virtually all social and economic progress in Tibet. The CPC emphasised the need for democratic reform as soon as the people of Tibet were ready to make that step.

All of this is explained in greater detail in the documentary which i linked in my other comment.

I recently posted about a documentary on precisely this topic. Please do yourself a favor and watch it as soon as possible. Be sure to watch both parts as the first deals mainly with the situation before the liberation while the second goes into the actual process of how the liberation happened. You don't need to take our word for it, listen to what people who were alive back then had to say and what they wrote.

The documentary references many western sources (too many for my liking in fact, i would prefer a less eurocentric view) both contemporary as well as modern, including some that have very little reason to be sympathetic to communists and some that were probably outright racists and sinophobes, and even these paint a very grim picture of pre-liberation Tibet. It also lets Tibetans themselves tell their stories. So it is by no means presenting an exclusively Han Chinese perspective.

If you still have questions, reservations or concerns afterwards there are more resources that we can recommend to you on this subject.

Well, they didn't do it in one day. There was a post a few days ago on a documentary about Tibet. It's from CPC-affiliated media, but it uses Western sources. Someone also commented a link to a CPC article talking about how they did it.

The PLA basically entered by signing an agreement with the theocratic government and they started growing crops and establishing schools, until reactionaries forces tried to drive out the PLA which invited support from the serfs to change the regime.

I agree our justification of meme-y, but actual Chinese could probably give a more nuanced explanation.

I'm far from an expert on the topic but I think this write up from the r/sino sidebar is a good starting point:

https://archive.ph/YtEPF

There is a lot of literature referenced in the write up that I'm sure would provide a more in depth understanding but I think it does a good job of providing a gist of things and also some of the historical context of the Tibet-China relationship. Another commenter pinguinu has also pointed out that the liberation of Tibet was a gradual process with many challenges, which I think is important to consider. This quote (which I got from the sino article) outlines both that and also the framework with which the CPC has approached these types of challenges well I think:

“As yet, we don’t have a material base for fully implementing the Agreement, nor do we have a base for this purpose in terms of support among the masses or in the upper stratum. To force its implementation will do more harm than good. Since they are unwilling to put the agreement into effect, well then we can leave it for the time being and wait. …” [Mao Tse-Tung, from Selected Works of Mao Tse-Tung, Volume 5]