As he verified on these tweets, the game is in the disc and you will install it from there, but Bethesda was not able to put the whole game in a DVD. Therefore, you will still be downloading the remainder of the game off of Steam afterwards.
It does make one wonder if game companies are considering the limitations of DVDs for 8th generation games. Could it be possible for future Steam retail releases to adopt Blu Ray? Is that even desirable in this day and age?