The sixth-century date for Codex Zacynthius (GA 040) was criticised by Parker and Birdsall back in 2004, and the Codex Zacynthius project has confirmed that the undertext was copied in the eighth century (at the earliest).
In addition, the identification of the scribe as Neilos also enables us to place the copying of the overtext (L299) in the period 1170-1190, thus twelfth-century rather than thirteenth-century.
Further discussion and evidence is presented in:
H.A.G. Houghton and D.C. Parker (ed.), Codex Zacynthius: Catena, Lectionary, Palimpsest (Texts and Studies 3.21). Piscataway NJ: Gorgias, 2020. ISBN 978-1-4632-4107-0 (www.tinyurl.com/czstudies)
H.A.G. Houghton, Panagiotis Manafis and Amy Myshrall (ed.), The Palimpsest Catena of Codex Zacynthius: Text and Translation (Texts and Studies 3.22). Piscataway NJ: Gorgias, 2020. ISBN: 978-1-4632-4105-6 (www.tinyurl.com/czedition)