In his later writings, Dogen set forth a number of very detailed texts on topics like reading and reciting Sutras, cleaning the temple, liturgy and ritual, general temple discipline, etc. He said all of this constitutes “practice”, even stretching the definition of “Zazen” to include these things. So no, “just sitting” is not the whole story after all, or if it is, it includes a very wide range of affairs.
Dogen also wrote a powerful text on the importance of moral discipline, and the Lotus Sutra is mentioned several hundred times in the Shobogenzo. In the fascicle “Taking Refuge in the Three Jewels” he calls the Lotus Sutra “The great king and grand master of all the various Sutras that Shakyamuni wrote, with all the others serving as its loyal subjects” (Kie bupposobo, Dogen 2.37).
Daily Sutra study took place from 4-5 pm without fail according to Dogen’s schedule for activities at Eiheiji Temple.
Lastly there is the famous and beloved text he wrote on “Instructions for the Head Cook.” Deeply moved by the wisdom of a temple cook who was also a monk but had little time for seated zazen, Dogen stressed that the position of temple cook should be considered a special high honor to be bestowed on a wise monk. In minute detail, he describes the steps for food acquisition, preparation, kitchen cleaning etc. and seems to regard the intensely focused precision of a cook who follows the rules to the letter as a not only valid but esteemed form of special practice.
So…in short, just sitting can mean more than literally just sitting. And why not? Dogen’s legendary word play gives him license to do whatever he wishes with mere words, no?