“What do you think, Susı̄ma, is form permanent or impermanent?”
–“Impermanent, venerable sir.”
–“Is what is impermanent suffering or happiness?”
–“Suffering, venerable sir.”
–“Is what is impermanent, suffering, and subject to change fit to be regarded thus: ‘This is mine, this I am, this is my self’?”
–“No, venerable sir.”
For any commentator (either a revered near-contemporary of the Buddha, BB, or someone here on DW) to know that Susima had achieved "insight knowledge" or "knowledge of the stability of the Dhamma", then either they are aware of some unexplicated textual detail, or they know something of Susima from another source.
If we held a poll here on DW with the above questions, many of us would give the same answers as Susima, wouldn't we? I can't see from the text alone that Susima "knows" any more than I would appear to know, notwithstanding any commentarial traditions to the contrary...