It is quite probable that the original inscription was carved in the name of Polibius. Olibi is, however, a testified genitive of the name Olybrius. The contraction PRIB, although certainly denoting a presbyter, is completely unique. The I should probably be read as "E"; the form PREB is quite common.
The dating of the second half of the 6th, or even more probably the first half of the 7th century follows that of RICG, which is, in turn, based on the linguistic analysis and the similarity to the other inscriptions from this period.
Surprisingly, Olybrius/Polibius is not listed under either of these names in PCBE Gaule, not even in the section containing fragmented names.