Timaeus is mostly devoted to natural philosophy.
Critias starts out rambling in a similar fashion.
Both are in the form of a dialogue, which would tend to point towards fiction for the purpose of discussion.
“Was and is there accurate information in them that lines up with known truths” ?
This is the important part.
Most classical literature no longer exists.
There could have been boring fact based works, that other authors referenced for an audience that would have read those works, but those works no longer exist, so all that is left are obscure references in “fiction”.
There is also the chance that certain information was kept from the general “plebs”, by referencing that information as mythological tales of uncertain accuracy, or as dialogue pretending that information is fake, so that only those with a wide breadth of knowledge, and access to better scientific knowledge, would be able to tell that that “fiction” that is presented, is actually truth.
This is a system that exists in the present day with Masonic imagery, and which seems to have existed far before “FreeMasonry” existed, which can be told by some of the imagery used, and the way that current imagery references older historically verifiable facts that were unknown for centuries or longer.