But I am! the more time passes, the more the antarctic melts!

They have taken into account the differences in above-sea-level and below-sea-level ice, as well as other things. If the ice is floating, I would agree with you re: simple displacement meaning no increase in sea level. However, this ice is not floating, it's resting on a land mass, so displacement does not apply, and from their wording they appear to have accounted for it too. Quoting:

We account for the volume of air contained within the firn in the nearsurface layers of the ice using modelled firn depth and density (Ligtenberg et al., 2011). We then calculated the mass of ice that could potentially contribute to sea-level rise. For parts of the ice sheet grounded on a bed above sea-level, this is simply the mass of ice lying between the ice-equivalent surface and the bed. For the part of the ice sheet grounded on a bed below sea-level, this is the mass of ice lying between the ice-equivalent surface and the flotation level calculated assuming ice density 917 kg m^-3, sea-water density 1030 kg m^-3 , and the GL04C geoid. Ice below the flotation level in the grounded ice sheet and in the ice shelves contributes to sea-level rise through its dilution effect on the ocean waters (Jenkins and Holland, 2007).
There is still substantial debate over the real potential for loss of ice in Antarctica to raise global sea level (e.g., Bamber et al., 2009b), and the second-order corrections required to evaluate the exact sea level change that would result from loss of ice in any particular area have been shown to be highly complex, involving as they do, crustal rebound, geoid modication (e.g., Spada et al., 2013), and thermosteric modi- cation of the oceans (e.g., Shepherd et al., 2010). However, this simple sea-level rise potential is nonetheless important in indicating the relative importance of Antarctica to sealevel change, and the degree to which our understanding of the subglacial landscape of Antarctica is convergent.

Think in 3D, not 2D.
This article appears to reference a decent study http://www.bbc.co.uk/news/science-environment-21692423 According to it, the average depth of ice in the Antarctic is around 2126m, (~6975ft, or ~1.3 miles!) At that depth, it would take the ice contained under a 1 square yard area to cover a football field with over a foot of ice. (6875*3*3 = 62275 cubic ft, 360*160*1=57600 cubic feet)

Oh yeah: that 2.1km average: it's apparently over a 12.295 million square kilometer area. 26.54 million cubic _kilometers_ of ice. while we're at it: surface area of the planet: 510,072,000 sq km (wikipedia).

So. simple math from there: 26,540,000/510,072,000 = 0.052km... or about 52m (170ft) for the planet if all ice in Antarctica melts. The article actually says potential equivalent of 58m, so an exercise to the reader to determine where the extra 6m comes from.. and how many cities that would affect.

BTW: Highly recommend seeing the movie Chasing Ice http://www.chasingice.com/ for a view of how fast the glaciers are changing. Netflix carries it.