The starting point for the astronomers' researcher was Caesar's carefully-documented phases of the moon and tides.
From the article:
"In 55 BC the full Moon came about three days before lunar perigee and about 3.5 weeks before the equinox, just as in 2007, so the key tidal factors would be virtually identical. On less than a dozen dates in the last 2,061 years has this match been so good. August 2007 offered the perfect chance to find out just where and when Caesar came ashore in 55 BC."