As I understand it, Dogen says that, at least for those new to seated meditation, there is no enlightenment outside seated meditation.
Well, to me that if true that's awful! No speech, no writing, no solidarity, etc.? I think it's this that is the stumbling block along the buddha way, for me.
Does it even have an answer?
One of Dogen's major teachings is 'practice and enlightenment are one' - what is important for Dogen is continuous effort in the dharma, and the basis of that effort is seated meditation. This applies to all practitioners, new and old alike.
Dogen tended to be disparaging of other schools, but if you want a less partisan Soto opinion you could read 'The teachings of Homeless Kodo' which acknowledges the value of other types of practice.
Perhaps you're reading a paraphrase or mistranslation. Wikiquote says ...
Zazen is the ultimate practice. This is indeed the True Self. The Buddhadharma is not to be sought outside of this.
... which is more or less what you're quoting or referring to in the OP.
But read item 2-22 (page 64 of 145) of this translation of Shobogenzo Zuimonki -- it says ...
Apart from sitting, there is nothing to seek as the buddha-dharma.
If you read the whole story I think the emphasis is meant to be on "not seeking", that it's "not to be sought" ... perhaps the lesson is that "seeking" is a symptom of desire, craving, aversion, vanity, etc.
The story at the end ...
Baso said, “How can you make a mirror by polishing a tile?”
Nangaku replied, “How can you become a buddha by practicing zazen?”
... might even imply the opposite about zazen: that the act, or outward form, of sitting is not sufficient.
