In the Platform Sutra, Hui Neng says: To concentrate the mind and to contemplate it until it is still is a disease and not Zen
But that's talking about forcing the mind to be still and that is a disease.
If Dogen's definition of "Zazen" equals "hishiryo", then it appears that Hui Neng is not criticizing Dogen's zazen, yes?
Does Dogen define zazen as hishiryo? Probably not since that would be pretty far along and can be see as just another experience.
If Dogen's definition of "Zazen" equals "hishiryo", then it would be correct to say zazen equals buddhahood, would it not?
My Zen teachers did not use the term hishiryo so the linguistic ramifications are not clear to me - but egoless awareness, thought before thought, these are characteristics of what we can experience in zazen and Zen training but it is not itself realization.
Before realization we sit zazen , during realization we sit zazen (although we might be startled) - after realization we sit zazen. All the while there is practice/enlightenment. Glimpses into our true nature can result in real realization and then we have to incorporate that into our lives.
But to answer what Dogen thought one would need to go through the Shobogenzo.
But, if you don't need to sit down in meditation to see your true nature, then what are you supposed to do, if you currently can not see your true nature?
You do need to sit down in meditation to see your true nature. Practically no one sees it otherwise and then you still need to sit zazen in order to deepen and refine that understanding. Zen is practice.
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche