My suspicion in the case of the Buddha’s omniscience is that the early canon (Pali and others) contained more than one position. Some suttas seem to be aware of the doctrine and others don’t seem to be, and sometimes the narrative exposes the different assumptions. When we try to force the entire canon to hold the same view, then these either/or debates ensue. The Abhidhamma tradition settled on interpretations to smooth things out. So, we have the Buddhist hybrid omniscience that he was omniscient insofar as he was paying attention.
It was interesting to look up the Chinese parallel to SN 54.9. The story in the Pali account seems toned down quite a bit. In the Samyukta Agama, bhiksus were killing themselves using a variety of means (knives, poison plants, hanging themselves with rope), until there was a “certain Deer Park ascetic” who is convinced by a bhiksu to help him kill himself with a knife. Then Mara tells the ascetic, “Oh, what a good thing you have done, releasing that good sramana to Nirvana!” Then the ascetic is convinced to continue doing this believing they were attaining Nirvana. So, it escalates to mass assisted suicide. He kills more bhiksus who come and present themselves to be released.
But when the Buddha comes out of his seclusion, it’s essentially the same. He asks Ananda why the congregation was smaller, Ananda explains what had been happening, and asks for a teaching. So, the Buddha teaches them about breath meditation.
It’s seems a story about teaching the right practice to treat a person’s particular tendencies.