Sleeping on things always does one good. I was quite exasperated at my complete inability to comprehend Ven Dōgen in that particular passage.
I think I might actually get now how Ven Dōgen's thought is meant to be understood in that quote, and like I suspected, for me it was a language problem, if indeed this was the misreading I was making. In light of correcting such a misreading, I would read the passage:
As the grasses, trees, etc, are synonymous with mind, they are synonymous with (the) sentient being.
Or "(your) sentient being" if we are to dip into the terminology of ātmavāda for just a moment for attempted clarifications sake. Now them having Buddha-nature makes sense, at least from an intellectual angle. If the above it right, I can at least follow his train of thought. Which would be an improvement for me.
Like this is the uncreated, like this is that which is difficult to realize, with no moving, no bending, no dying. Utterly lacking secretions and smothered in the dark, it is the island shore. Where there is ferrying, it is the crossing. It is dependency's ceasing, it is the end of circulating transmissions. It is the exhaustion of the flame, it is the ending of the burning. Flowing openly, pure and cool, with secret subtlety, and calm occultation, lacking ailment, lacking owning, nirvāṇa.
Asaṁskṛtadharmasūtra, Sermon on the Uncreated Phenomenon, T99.224b7, Saṁyuktāgama 890