Sudden enlightenment described in the suttas

Your question looks like the question "why the scientist can understand the relativity theory of Einstein, it is unbelievable!".

This is the reason why the theras told their story inside KN thera/therī-gāthā and KN thera-therī-apādāna at 1st saṅgāyanā of thera-vāda, because they want to let us know that they practiced themselves so long time ago more than you see their enlightened moment in sutta.

So, you and me, who have not enough practice-experience like those theras, still not enlighten after read many suttas.

For the present life answer see: https://unmixedtheravada.blogspot.com/2017/09/yugganaddha-patipada-abhinna-obhasa.html

Question:

A lot of suttas mention the case of sudden enlightenment, during the time of the Buddha. Do these cases speak only of stream-enterers, or can they even be full arahants?

For me it's unbelievable, that either others or I must simply (!!) understand impermanence, in order to become an arahant.

Thanks