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.