Half-giants are a hybrid creature from the distant past, historical interbreeding of giants and something else. In [ citation needed] , the World of Warcraft vrykul of Northrend are the only group to have been described as such.       They thus may not necessarily be the result of interbreeding between  giants and humanoids, but rather a midpoint in an evolution of giants into humanoids.
The " Story So Far" article for mentions that the half-giant vrykul are "descended from giants". However, Wrath of the Lich King establishes that the vrykul were created by the Chronicle Volume 1 titan-forged keepers, who are also occasionally referred to as giants, through the  Forge of Wills, along with the  earthen and others. So while they are not descended from giants in a literal sense, one could still say the vrykul trace their origins back to giants.
It is also likely that vrykul being described as half-giants represents early development material that was later left out of the final release, much like the reference to vrykul being "vampiric". Alternatively, "half-giant" may simply be referring to the vrykul's large stature.