Besides, record keeping from that era was not exactly best... and people are trying to extrapolate events based on a very small frame of reference from a culture they had little understanding of.
In Trek, you have a universal translators, computers and algorithms capable of taking small information and extrapolating a huge chunk of it with incredible precision, historical databases from hundreds/thousands of alien species which may have experienced similar progression as humanity in history... not to mention a sort of 'standardization' (a common language) which makes things even easier in regards to mutual interaction, cooperation, etc. (and incredible data redundancy on individual planets, starbases, ships, etc. - too many to count - not all of which would be lost to the Burn).
Mind you, even today we are using computer algorithms to gain a better idea of what happened thousands of years ago based on fossils, bone remains... what kind of diet the people had, etc... it gives us a better insight into their culture and how their lives unfolded.
Moving forward, it will be increasingly easy to store existing data and knowledge with massive redundancy for the future - because we have far superior methods of doing so than the past did and this ability will only improve as time goes on.