Last season, the most frequent line up by far (495 minutes compared to next closest at 140 min) was a line up of Irving-Smart-Tatum-Morris-Horford. This group of 5 was also the top 5 on the Celtics in terms of average minutes per game. To me, that is a pretty traditional line up. You can quibble as to whether Morris is a big or swing he is small for a PF but certainly does not constitute "going small" to have him on the court with Horford.
Looking at this another way, the Celtics played about 66% of the time with two bigs on the court last season (two of Horford, Morris, Baynes, Theis). I am not sure what everyone thinks he is reinventing here? I think he understands full well that you are better off having two bigs on the court most of the time. His favorite line up last season lined up just fine with the traditional PG-SG-SF-PF-C.
Yes, there was plenty of the time where they went “small” where Tatum (mostly) or Hayward (sometimes) played without a second big alongside Horford. But I think the problem there is talent, not some quest to redefine basketball. He played Horford Morris, and Baynes about as much as he could so to get more size on the court would have meant more of Theis, Ojeleye, Yabusele, or RWilliams. So he elected to play Tatum out of position as the lesser of two evils.
So I guess I agree with the OP that it is wrong to "go small" in an extreme but disagree that there is evidence they Celtics tried to do this last season (or ever).