Without context, its tough to say, but I don't think its that outlandish.
I honestly feel a little uncomfortable with the idea of KG and Ray going down in history as true Celtics, the same way I feel uncomfortable with thinking of Wilt as a Laker, or Barkley as a Sun. I love what they did, but where it was only a fraction of their career, I just don't think I look at it the same way as if they were there for most of their career.
With Pierce, obviously, he is a career Celtic, but (again, need to see the context of the comment), I can see an argument where Kobe is more integrated into Lakers history than Pierce is in Boston.
For one thing, there are the championships. For Kobe to span two great championship runs is certainly impressive.
But, for Pierce, I think people seem to forget how much he had fallen off the radar in Boston before the Big 3 got here. He not only became close to irrelevant in the Boston sports scene for about half the decade, he became the symbol for futility for a while. There was a huge contingent calling for him to be traded, and with bandagegate against the Pacers being the height of it, he was just not looked on very highly among Boston sports fans.
Of course, he completely redeemed himself when the Big 3 showed up, but I am not sure I would put him in the Boston Sports pantheon, the way Kobe probably belongs in the pantheon in LA.