Brian Robb points to the fact that he expects the Johnson deal to be a S-n-T, presumably with the Rondo TPE and a little something thrown in for TOR. This would allow the Cs to keep the smaller 7.7mil (still very useful) exception that remains.
With Ainge clearly valuing flexibility, this is what makes the most sense. It never made any sense to get totally under the cap unless they were adding a Love or Aldridge type of player. Once it became clear that they were not going to land the proverbial big fish in Free Agency, incremental improvement became their M.O. Getting Johnson and Jerebko signed to essentially 1 year deals (given the non-guaranteed 2nd years in both cases), they increase their depth, slightly improve the team as constructed, and still maintain their flexibility to trade.
In my opinion, this was always the most likely course. Their cache of assets allows them to be viable contenders for anyone that comes on the market. The downside to this is that so far, their assets don't seem to be valued as high by other teams as we would like. But it remains incredibly early in the off-season, so it seems a little silly to be evaluating what clearly is not a finished product.