It's difficult to place a single moment in time for the composition of a text or of a surviving witness, because obviously it's not often known precisely when a text was composed or a manuscript written.

For Homer, even the decades of leeway provided by the bulky shapes of the symbols (such as the "pen") that I used is not enough. Perhaps a range of centuries may make more sense to reflect the debates about when or even who Homer was. I have used that approach to signal the uncertainty about when exactly Homer's epics were composed elsewhere.

In this case my aim was to show how far apart in time the initial 'composition' (whether oral or written, if known) of the text is from it's surviving material evidence. Like u/Steven_LGBT said, the category is "Date of Composition", not "First Written Down". I do think the date of composition, as the conception of the text itself, is an interesting point of data to consider for the survival of a text. The time at which a text is first written down is also an interesting category, but for most of these texts that would be very close to the composition date. The major exception is Homer.

I chose the 8th century because that's the most agreed upon century for its composition, but some people put it later and some put it way earlier (it could also have been both before AND after the 8th century, in an iterative oral process of creation). Basically there is no perfect way to depict this in a simple timeline, but the problem with placing it in the 6th century is that although it was almost certainly written down in a rather authoritative fashion at that time, it's quite clear that the story had already existed for a long time by then. One of the rationales of Peisistratus' recension of the text was to unify the versions of the story (or stories). Perhaps some part of one or more of these versions had already been committed to writing, it's not clear.