This is such an excellent point. I think some gifted kids are going to have social issues no matter where they are because they are quirky or overly emotional. These are common gifted traits, and they both make getting along with others a bit tricky, regardless of how old the child is compared to how old the other children are.
In the grade accelerated child I know who had the bumpiest time with social adjustment had a strict and I felt overly controlling mother. The mom insisted on rules for her in line with her chronological age rather than her peers' age, and this just exacerbated the problem. None the less, it is quite impossible to say how the child would have done without acceleration. She would have been less engaged with her school work, and may have still had the mom drama that naturally led down a path of rebellion that we ALL want to spare our kids from.