'The piece was discovered by Giovanni Varelli, a PhD student from St John’s College, University of Cambridge, while he was working on an internship at the British Library.'
It dates from around 900. Read more about it here. Polyphony was being discussed at this time, but the earliest written notation up to now was from a century later.
It is organum, but not a simple matter of the one part being five notes below the other; the interval varies. It undermines the assumption that the earliest polyphony was organum carried out mechanically, with one voice following the other at a fixed interval.
The text is:
Sancte Bonifati martyr inclite Christi te quesumus ut nos tuis precibus semper gratiae Dei commendare digneris.
O holy distinguished martyr of Christ, Boniface, we beseech you that you should always see fit in your prayers to commend us to God's grace.
With the name of 'Vitus' instead of 'Boniface' the text occurs as the single antiphon for the Vespers psalms for Vitus in two Austrian MSS of the 14th century, one of which has images on the Cantus website. This was probably the antiphon for Vespers for St Boniface.