What was done
The author calculates the sea level change that would likely accompany a melting of the global ocean's inventory of submerged and sequestered methane hydrate, which upon dissociation looses 21% of its volume.

What was learned
The author concludes that the combined effects of methane hydrate melting and subhydrate gas release would lead to a significant decline in global sea level, conservatively estimated to range from 0.1 to 1.5 meter, but which may in the past have produced sea level drops of as much as 25 meters.

What it means
In the words of the author, "such a mechanism may offset some future sea-level rise associated with thermal expansion of the oceans," such as could occur in response to continued global warming.  Indeed, he states that the estimated sea level change is "of the same order of magnitude [but of opposite sign] as those associated with thermal expansion of the oceans, melting of nonpolar ice, and melting of the West Antarctic ice sheet."  Hence, it is not at all clear what impact these several simultaneously occurring phenomena might have on mean global sea level.  Maybe the Maldives might even grow in size!