"What's past is prologue" means that the past tells us why things are as they are now. "Prolix" means wordy, and long winded. "Past is prolix" means the past just won't shut up.
There is a clear connection to Shakespeare's "The Tempest" I ii, which gives Antonio the lines:
We all were sea-swallow'd, though some cast again, And by that destiny to perform an act Whereof what's past is prologue, what to come In yours and my discharge.
We all were sea-swallow'd, though some cast again, And by that destiny to perform an act Whereof what's past is prologue, what to come In yours and my discharge.