It was not a fluke.
It wasn't a sign that they were a contender, either. This year's team is probably closer to last year's team than last year's team was to being a championship team.
This is a non-sequitur. I didn't argue at any point that last year's team was a contender.
That's an entirely different matter from saying that last year's team was competitive, that the playoff run was not a fluke, and that it therefore reasonably created expectations that the team would also be competitive this season.
Just to give a sense of how big a difference we've seen this year:
Over the course of the regular season, bubble, and playoffs last season, Kemba, Smart, Tatum, and Brown spent 567 minutes on the floor together without Gordon Hayward.
In those minutes, the Celtics scored 111 points per 100 possessions, while allowing 105 points per 100 possessions. That's a +6 differential, which is quite good.
This season, those same four players have shared the floor in 292 minutes. During those minutes, they've scored 122 points per 100 possessions, while allowing 125 points per 100 possessions, which is a -3 differential.
That's a net change of -9 points per 100 possessions. They've gone from quite good to quite bad.