(not a scientist, but was skeptical of this statement given the general "incompressibility of water")

From looking at the phase change chart[0], you don't really get into the "interesting" stuff as far as pressure goes until pretty deep (something like 9-10km, from looking at some sample number of pressure at various depths [1]). Given that they indicated in the article that this is under 1.5km of ice, I don't think that's deep enough to have pressure significantly change it, but would be happy to learn that I'm not correct in my reading of this :)

0 - https://physics.stackexchange.com/questions/60170/freezing-p... 1 - https://www.earthlearningidea.com/PDF/189_Pressure_rock.pdf - table on page 2