一竅通千竅 One Aperture Opens a Thousand
長老聞得，亦開茅塞。正是：长老闻得，亦开茅塞。正是：
逐句解 Line by line
The elder, hearing it, had his ignorance too unblocked.
開茅塞 is to clear the choking weeds — Mencius's image for a mind blocked by disuse, and still the ordinary Chinese way of saying somebody has finally seen the point. 聞得 takes 得 as de; 長 is zhǎng.
Just so:
The tag introducing the couplet that sums up the night.
When the principle is clear, one aperture opens a thousand; expound the No-Birth, and that itself is immortality.
Two traditions in one line. 一竅通千竅 — one aperture opens a thousand — is the Daoist image for a single insight letting light into everything, the apertures being the body's channels. 無生 is the Buddhist Unborn, that which neither arises nor perishes. And the couplet welds them: grasp the Buddhist truth and you are, it says, a Daoist immortal.
A brief hinge. Tripitaka, having taken in both Wukong's alchemy and Sha Monk's completion of it, is himself enlightened, described with the old image of choking weeds cleared from a blocked path, and the narrator caps the moment with a summarising couplet.
The couplet states the upshot in one balanced line and does it by fusing the two traditions the discourse has been braiding: the Daoist single aperture that opens a thousand beside the Buddhist Unborn, with the striking claim that to expound the one is to become the other. The moment of shared understanding is offered as the real fruit of the moonlit night — just before Eight Rules arrives to puncture it.