The combined canonical information about the history of Terra says so.
Based on DPG's work, the Solomani, especially The Confederation government, SolSec and Party members have absolutely no qualms about suppressing, or distorting history in order to further Party agendas or policies. The 'Gene War' provides a direct example of such tampering within the setting. In a realistic TU, there are sure to be other examples.
As far as collapses go, can anyone say that there were no regime changes inside the Solomani sphere of influence between the Terra of IY 1106 and Earth 1969AD? Political upheavals, even peaceable ones, will affect what sort of records will be passed along and which will be suppressed or run through a level of 'spin'.
So what if data is saved....how much of it be guaranteed to be accurate by 'OTU' historians?
http://www.cs.dartmouth.edu/farid/research/digitaltampering/
Given that the Solomani appear to support ethnicism and nationalism, how many of the saved records are tainted by the effects of spin and propaganda that exist to promote such nationalistic/ethnocentric beliefs?
http://en.wikipedia.org/wiki/Historiography_and_nationalism
And this opinion is not even taking into account the numerous pseudo-history writings or crackpot theories of events, or urban legends, etc.
Sure, Terrans of IY 1106 might know things about 1969AD, but very very few will know anything with any amount of accuracy or confidence. The masses will only know what the media outlets textbooks, 'everyone knows that! We learned that in school!*.....', popular media and blatantly slanted 'news' programs, tell them, which is probably whatever the government agencies allow the media outlets to say.
*intoctrination and instruction to be good party members + a dose of nationalism, maybe?
incomplete and/or inaccurate information gives more wiggle room for stories anyways.