As far as I can judge, an important part of the sea level cycle implies the changing ratio between coastal to deeper water single cell organism. The same technique I have seen being used here: see chapter 6 Eustatic sea level rise
Edit: futhermore it may be noted that we may have had an event, known as melt water pulse 1A, which also implied a quick sea level rise lacking evidence of a clear source. See this thread
Furthermore the speculation of earth geometry changes related to eccentricity, again, is for the account of the authors. The only thing that matters is how accurate the sea level fluctuation has been identified/calculated.