Shared Sequencers are a different concept and, depending on the implementation of the sequencer and the rollup, will have more or less extractable MEV. This is independent of the DA-Layer, so it does not matter if it is on Celestia or Avail.
When the DA-Layer “acts” as a shared sequencer for based rollups, for example, it depends on the DA-Layers implementation and the Rollups’ interpretation. Read here on Based Rollups and an implementation Idea of them here.
In short: Any Based Rollup will leak MEV to any DA-Layer if the DA-Layer can capture anything from reordering. 