The 2019 team underperformed by a lot. Clearly, they were a better team on paper than 78 wins. They were pretty close to the same team as 2018, and it's harder to "elaborate" on the massive drop in wins between 2018 and 2019 than 2019 and 2021.
You also have to look at the strength of other AL teams. The Astros are way worse than the 2017-2020 versions. The Rays look worse on paper, but I never count them out.
The CWS have improved. The Twins and Angels should be good. The Guardians and A's may compete, but the rest of the AL is in bad shape- worse shape than the Sox.
I'm not going to say projecting 78-82 wins is wrong. I can certainly see that happening, but I think our 2021 team should be much better than the 2020 team which projected to 65 wins.
We did end 2020 going 12-9, which included wins vs NYY, ATL and TBR, and 2020 saw 60- 80% of our rotation on the IL at any given moment.