Um, yeah, except for the whole, "the cap is rising dramatically so the Celts will still have space to add impact players in the future."
The argument for adding Harris and Monroe, basically, is that you have to build something before the really good players will want to come to your team and actually stick around. The Celts probably won't get the chance to do it through the draft, so they have to try and do it through FA or trade.
Harris and Monroe are second tier players, at least right now, but they'd immediately be the best players on the team and provide a framework for future success.