大般涅槃經義記 4.207

4.207

Below, it clarifies [the sūtra's] lack of efficacy according to the time of decline, first with an analogy, then the application. From "Just as a mosquito's oil cannot moisten the mind-ground of sentient beings" onward, it takes the opportunity to clarify the marks of the complete extinction of the Dharma. This sūtra being the first to be extinguished and no longer circulating in that place is called "sinking into the earth." From "You should know..." onward clarifies that when this [sūtra] is extinguished, all the other sūtras will be extinguished as well.