There is a difference between knowing “who the Buddha was” (where the person was born, how old he became and so on) and “what he is/was”, where, as I understand it, the deeper meaning of Buddha (bhava+uddha or “having extinguished further existence”) comes in. So, it refers to the qualities of “being free from defilements”, and also free from death and decay, in other words referring to the timeless quality of Nibbana. These qualities are independent of a person and still exist and always will. Of course, here I am referring to “Buddha” as a quality, it can also be done with Prefix “samma sam”(buddha).
There are some intellectuals who know a lot about the person(for example there are some PhD in Buddhism – not that this is anything wrong or bad at all, it is certainly helpful) but unless there is an understanding of the 4NT, it is not really clear what he really came to teach and what his task was (because one does not see the real problem, i.e. that “wanting to play video game” (attachment to sankhata) is root problem and not “not being allowed to play the game” (dukkha vedana).
That’s why it is said that only a Sotapanna can start to see some of the the “supreme qualities” of a Buddha (the unique qualities or culmnination of everything “pure”) due to the firstglimpse” of the 4 Noble Truths.
Here this article of Lal might be helpful. It might explain it in better words.
