Funny fact, that 2007-08 team shot more three pointers in the regular season and shot a higher percentage of three pointers than any other champion in NBA history. So I think the OP is incorrect. This team has been a team that was always a three point shooting team.
That is to be expected, as the 3 point shot is getting progressively more important in the NBA game (full adaptation to the "new" defensive rules, faster and bigger defenders, focus on avoiding dribble penetration -> more congestion on court -> less post isos, less dribble drives -> more ball movement and shooting). SAS in the year before shot three less three point shots (at the exact same percentage, curiously). The Celtics opponents in 07/08 shot 29 less 3 pointers. This season, the Celtics are actually taking less 3 pt shots than their opponents. The difference for last season Lakers was merely 3% on 3pt shots taken (and the Lakers shot at a 0.361%).
I don't think the Celtics are particularly reliant on the outside shooting - and considering the minutes that good shooters play, they probably should be shooting more.