大般涅槃經義記 8.138

8.138

自下第三明前愚人所見非中，「見一切空，不見不空，不名中」者，約空、不空以顯非中。二乘唯見生死法空，不能見於涅槃不空，偏故非中。乃至「見於一切無我，不見我者，不名中道」，約我、無我以顯非中，超苦無常故言乃至，義同前解。

From here below is the third [part], clarifying that what the former foolish person sees is not the middle: "Seeing all as empty, without seeing the non-empty, is not called the middle" reveals non-middleness with reference to the empty and the non-empty. The two vehicles only see the emptiness of the dharmas of saṃsāra and cannot see the non-emptiness of nirvāṇa; being one-sided, it is therefore not the middle. Up to: "Seeing all as non-self, without seeing the self, is not called the middle way" reveals non-middleness with reference to self and non-self; skipping over suffering and impermanence, it therefore says "up to"; the meaning is the same as the previous explanation.