All of the basic Monster Manual giants have a melee weapon attack with reach of 10′, which essentially comes from their Huge size. But the Stone Giant has a reach of 15′ with it’s greatclub, which is farther than the reach of a Hill Giant’s greatclub, the Cloud Giant’s morningstar, the Frost Giant’s greataxe, or the Fire or Storm Giant’s greatswords. The huge Cyclops, with 1 more hit die than the Stone Giant (thus at least the same size), also has a greatclub with reach 10′.
I don’t see any reason for this. The Stone Giant is not depicted as having disproportionately long arms, and it’s only in the middle of Giant heights; Frost, Cloud, and Storm Giants are all taller. Nothing in the description mentions Stone Giants carrying a particularly long club or having a talent for swinging it further. Rather, the description talks about their skill at throwing rocks. The only monster I know of that is specified as having longer reach attacks ("long-limbed") is the PC version of the Bugbear in VGtM.
None of the previous editions (AD&D, 3.5 or 4th) give the Stone Giant extra reach with a club.
So my suggestion is that this was a typo that was never caught & corrected. The reach of this attack should be 10′ like that of any other huge Giant.