Can progressive aspects express the ongoing effect of an activity?

"The Sorting Ceremony will take place in a few minutes in front of the rest of the school. I suggest you all smarten yourselves up as much as you can while you are waiting."

Her eyes lingered for a moment on Neville's cloak, which was fastened under his left ear, and on Ron's smudged nose. Harry nervously tried to flatten his hair.

"I shall return when we are ready for you," said Professor McGonagall. "Please wait quietly."

She left the chamber. Harry swallowed.

"How exactly do they sort us into houses?" he asked Ron.

"Some sort of test, I think. Fred said it hurts a lot, but I think he was joking."

Harry's heart gave a horrible jolt. A test? In front of the whole school? But he didn't know any magic yet -- what on earth would he have to do? He hadn't expected something like this the moment they arrived. He looked around anxiously and saw that everyone else looked terrified, too. No one was talking much except Hermione Granger, who was whispering very fast about all the spells she'd learned and wondering which one she'd need. Harry tried hard not to listen to her. He'd never been more nervous, never, not even when he'd had to take a school report home to the Dursleys saying that he'd somehow turned his teacher's wig blue. He kept his eyes fixed on the door. Any second now, Professor McGonagall would come back and lead him to his doom.

(Harry Potter and the Sorcerer's Stone)

The progressive form doesn’t seem to denote Fred’s ongoing joking activity for of course he's not joking now, but the imperfective effect of the activity. That is, he joked in the past but still now Ron hasn’t got the conclusion that his words were only joke. Is this what the aspect is saying? I mean can progressive aspects express the ongoing effect of an activity?

If that's not the case, does the aspect express the internal viewpoint: Ron reminds of the event not as an outsider but an participant in it?

Answer

We might call this a descriptive use of the progressive. Telic verbs like joke or explain or lie ordinarily express something done-to-the-end which brings about a change of state: a joke excites laughter, an explanation brings about understanding, a lie brings about a misunderstanding or false belief. When such a verb is cast in the progressive it sometimes implies imperfectivity, an action—joking, explaining, lying—in progress but not yet completed; at other times however it is used to describe an action, to characterize it as possessing the quality of a joke, an explanation, a lie.

If he says that, he’s lying.
He wasn’t ordering you to do it that way, he was explaining why it should be done that way.
He wasn’t serious, he was just joking.

The same recategorization may be observed with some atelic verbs. BE, for instance, is ordinarily a stative, but if cast in the progressive it describes behavior rather than asserting existence.

John isn’t really a jerk, but he’s certainly being one tonight.

Note that in my ‘serious’ and ‘jerk’ examples the progressive construction is contrasted with a stative copular construction, not with a perfective construction.

My guess—it’s no more than that, and if anyone has a better understanding of this I’d be glad to hear of it—is that this use is a matter not of the Viewpoint Aspect, imperfectivity, conferred by the progressive construction, but of its Lexical Aspect: stativity. That is, the progressive is employed because it attributes a state, a quality, to the subject rather than an action.