Further down in the linked text the Mahathera mentions various suttas that the commentators used to put together this 5-fold niyama system. He also said it was done mainly to refute the creator-god notion.
Why the authority of this niyama scheme should be questioned as fully "Buddhist" puzzles me. Mahayana is chock full of texts by Nagarjuna, Dharmakirti, Asanga et al, which are accepted as the simple unpacking or organizing of Buddha's teachings. So why not give these non-Mahayana commentators the same respect?
One should refrain from biased judgments and doubting in fathoming the Buddha and the Dharma of the Buddhas. Even though a dharma may be extremely difficult to believe, one should nonetheless maintain faith in it. Nagarjuna