You could be right, I don't know. And reading Ainge's quotes from when KG was signed, and when he was retired, I now think it's probably more likely than not his number is retired. But I'm not really a fan of it, should it come to pass. My stance on retired numbers is that it's reserved for players that indelibly associated with a particular franchise in a positive way, and in the vast majority of cases that will mean A) playing the bulk of their career with that particular team, including at least most of their prime; and either B) being so notably outstanding during their time with the franchise as to almost single-handedly bring a sense of honor (?) and respect to the franchise purely by wearing the uniform (e.g. Ted Williams, John Hannah, Ray Bourque); or C) being hugely instrumental in bringing glory to the franchise, typically through winning one or (ideally) multiple championships (e.g. (surely) David Ortiz or Robert Parish). Oftentimes, B and C will overlap (e.g. Larry Bird and, (surely) Tom Brady), and that's the best of all.
I think Garnett misses out on "A" and, given the Celtics ridiculous track record, probably "C" as well. He was absolutely instrumental in winning a Championship, but if we're grading on a scale and comparing him against other Celtics legends, he doesn't quite measure up. Pierce, otoh, clearly meets A and also meets C more than Garnett given his role in bringing the team back to respectability in the early 2000s.
Now, I realize all of these standards are somewhat arbitrary and even political on some level, so I respect that you may disagree.