This seems like a non-story. Tesla is selling older "demo" vehicles. They're clearly marked as such and discounted, although not as much as the article would like. It's equivalent to buying a floor model at a furniture store.
A Frankensteined 2021 model with a 2017 battery pack is very unusual, and would have had to be specially built. Who knows what else is different about the car. Is it even a 2021 car? Or is it an old car they are calling 2021 since it wasn't registered.
Ignoring the range/age impact, getting service done may cause problems when technicians expect parts to match the model year of a car. Although Tesla has many unannounced engineering changes within the same "model year", so maybe it's a non issue. Will their software updates account for mixed model year parts?
For normal ownership and usage it sounds like a terrible prospect, with several uncertainties.
Could be a unique future collectable though.