It just means his mind was made very clear and still by listening to the Buddha's Dhamma talk on the preliminaries. It gladdened him and suppressed the hindrances enough for him to gain personal knowledge of the Four Noble Truths when they were revealed to him.
As to your most recent question. You quoted one in your OP.
I think you're attempting to reify something that doesn't need to be given that much attention. Ask yourself, what's it like when your mind is clear? What's it like when you're distracted.
I've found that after a particularly good samatha session, my mind is very clear and concentrated and it's easier to direct my thought towards Dhamma analysis. It is possible to still one's mind simply by paying close attention to a dhamma talk that one finds gladdening. Throughout the Suttas we find the Buddha giving these preliminary talks, in order to gladden and rouse the minds of the Bhikkhus or whoever he's talking to, for the express purpose of giving them a deeper talk on the more fundamental aspects of Dhamma that they might not otherwise have understood if they were not prepared in mind.
"For a disciple who has conviction in the Teacher's message & lives to penetrate it, what accords with the Dhamma is this:
'The Blessed One is the Teacher, I am a disciple. He is the one who knows, not I." -