I think we have to read through the text and assume that Saccaka’s tone throughout most of the conversation is cheeky and sarcastic.
Later in the conversation he says:
“This is a matter about which Master Gotama can be trusted, as an accomplished and fully enlightened one should be. But does Master Gotama recall sleeping during the day?”
and after the Buddha answers, and says he is able to sleep mindful and fully aware, Saccaka says:
“Some recluses and brahmins call that abiding in delusion, Master Gotama.”
It is easy to imagine Saccaka’s words in these cases being delivered in a biting and mocking tone. The evidence that they in fact were delivered in that tone seems to come at the end, when Saccaka, now sincerely impressed, says,
“It is wonderful, Master Gotama, it is marvellous how when Master Gotama is spoken to offensively again and again, assailed by discourteous courses of speech, the colour of his skin brightens and the colour of his face clears, as is to be expected of one who is accomplished and fully enlightened."
… and contrasts Gotama’s response with that of other supposedly accomplished teachers. It looks like Saccaka is now convinced that the pain of rough and offensive speech has indeed not “invaded” the Buddha’s mind, despite how aggressively he has been “assailed” with it, and that this cheerful and equanimous response has not been found in other teachers who have been assailed in the same way.
Since sutta transmission was originally oral, I wonder if the the early traditions of recitation included standardized tones of voice and affective coloring that were lost when the suttas were committed to writing.