The Celts are playing elite, perhaps championship level defense, but their offense remains below average. An upgrade will be necessary.
Not sure what statistic you are using for offense… As of now, league average is 104.1 points per hundred possessions, while the Celtics are at 104.9 per hundred, so by that measure they are slightly above average. My source for that is Basketball Reference, and I know that numbers seem to vary depending on the source. Incidentally, according to the same site, the Celtics are now second in defense. It's early in the season, so three games makes a big difference.
As for the question in the thread, I would suggest that the Celtics have an additional advantage to their chemistry, namely their depth.
I for one would love to see if a team with no stars but a lot of above average players with great chemistry could win a championship – but we are unlikely to find that out with these Boston Celtics in the next few years.