I don't get the distinction Ainge is making. What is "rebuilding", if not tearing down a previous team and attempting to build toward a championship?
I think he sees this current squad as having a good shot for the 8 seed. As in we already have a foundation and we just need to "improve" rather than "rebuild."
If you only spend one year out of the playoffs, I don't think that can technically be considered rebuilding (or at least I personally wouldn't). That's closer to "reloading."
My guess is that's just his way of saying we're trying to make the playoff this year, rather than continuing to rack up consecutive lottery picks like a traditional "rebuild" (OKC, etc.)