You can read this article by Western media to get a different perspective: http://www.cbsnews.com/news/chinas-leader-talks-to-60-minute...
edit: I'm overestimating Chinese media freedom here. Furtive word of mouth and descriptions from people who have travelled outside of China are probably a more accurate way that Chinese people have learned about this facet of their own history - am I right?
They are more likely to be motivated to give a biased recount against the government as a way of expressing their resentment with the government. They also often have supports from people/organizations who are eager to find ways to paint Chinese government black (for whatever reason they might have). (Think the situation of Snowden)
Why? Were they everywhere? Did they witness everything? Do they remember perfectly?
Edit: Typo
They where [cis] not everywhere, but the incident was pretty widespread and every major city was having its "gathering". (Yes, western media does not care about that but that was what happened as they recounted). There is no way that someone living a major city did not know about it.
> Did they witness everything?
Regarding the actual facts, people have friends or colleagues who were working in Beijing so they could discuss it privately.
> Do they remember perfectly?
They don't have photographic memory but I am sure an incident like this would be memorable.