Well, I don't know what they're planning, but ISTM that if they divide the storage area they can greatly extend the time at which they're generating energy in exchange for nearly halving the peak generation capability...and without much pumping (which adds an additional inefficiency or three).
OTOH, the amount of energy that can be generated by water stored at a particular height depends on the fall distance. So the potential generation capability will vary a lot as the tide changes. Maybe some of the inflow could be used to drive a hydralic ram to lift some of the water higher than max high tide level. But that *does* introduce additional inefficiencies.
All in all, I don't know, but it looks pretty iffy.