Why COULD Harry see Thestrals following Cedric Diggory's death?

Question

Pursuant to my question here, asking why Harry couldn't see Thestrals immediately following Cedric Diggory's death, now I want to know why Harry could see Thestrals, as a re-read of canon shows that Harry actually did NOT see Cedric get killed. He had covered his face and eyes with his hands and his eyes were closed as Cedric was killed.

And then, without warning, Harry’s scar exploded with pain. It was agony such as he had never felt in all his life; his wand slipped from his fingers as he put his hands over his face; his knees buckled; he was on the ground and he could see nothing at all, his head was about to split open. From far away, above his head, he heard a high, cold voice say, ‘Kill the spare.’

A swishing noise and a second voice, which screeched the words to the night: ‘Avada Kedavra!’

A blast of green light blazed through Harry’s eyelids, and he heard something heavy fall to the ground beside him; the pain in his scar reached such a pitch that he retched, and then it diminished; terrified of what he was about to see, he opened his stinging eyes.

Goblet of Fire - Page 553 - British Hardcover

And:

‘Righ’, now, who can tell me why some o’ yeh can see ’em an’ some can’t?’

Hermione raised her hand.

‘Go on then,’ said Hagrid, beaming at her.

‘The only people who can see Thestrals,’ she said, ‘are people who have seen death.’

‘Tha’s exactly right,’ said Hagrid solemnly.

Order of the Phoenix - Page 394 - British Hardcover

According to the above passages, Harry technically did not see Cedric get killed, but witnessing a death is required for seeing Thestrals. So how is it that Harry was able to see Thestrals?

Answer 1

The obvious answer would be that the literal act of seeing isn't necessary, rather that it's experiencing someone's death that is key.

Rowling somewhat backs this up:

Harry did not see his parents die. He was one year old and in a cot at the time. Although you never see that scene, I wrote it and then cut it. He didn’t see it; he was too young to appreciate it. When you find out about the Thestrals, you find that you can see them only when you really understand death in a broader sense, when you really know what it means.

Someone said that Harry saw Quirrell die, but that is not true. He was unconscious when Quirrell died, in Philosopher’s Stone. He did not know until he came around that Quirrell had died when Voldemort left his body.

Then you have Cedric. With Cedric, fair point. Harry had just seen Cedric die when he got back into the carriages to go back to Hogsmeade station. I thought about that at the end of Goblet, because I have known from the word go what was drawing the carriages. From Chamber of Secrets, in which there are carriages drawn by invisible things, I have known what was there. I decided that it would be an odd thing to do right at the end of a book. Anyone who has suffered a bereavement knows that there is the immediate shock but that it takes a little while to appreciate fully that you will never see that person again. Until that had happened, I did not think that Harry could see the Thestrals.

Source

She does refer to Harry "seeing" Cedric's death, but also explicitly says it's the act of appreciating the death that makes the difference.

Personally I think it's something that slipped through the gaps (nice catch!), but I also think it's explainable in-universe.

Answer 2

He mightn’t have to see the moment Cedric died to “see death”.

Harry didn’t see the moment Cedric was killed, but it’s never been said that actually seeing the person die was a requirement for being able to see Thestrals, just that the person has to “have seen death”. It’s never said that they have to see a death take place, though. Harry can be said to have seen death - he looks at Cedric’s dead body, and though he didn’t accept what he was seeing at that moment, he did recognize the body was lifeless.

“Cedric was lying spread-eagled on the ground beside him. He was dead.

For a second that contained an eternity, Harry stared into Cedric’s face, at his open grey eyes, blank and expressionless as the windows of a deserted house, at his half-open mouth, which looked slightly surprised. And then, before Harry’s mind had accepted what he was seeing, before he could feel anything but numb disbelief, he felt himself being pulled to his feet.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 32 (Flesh, Blood and Bone)}}

It’s never made clear what “seeing death” is defined as. For example, it’s never mentioned if having a loved one die and emotionally processing it but never actually having seen the dead body would be enough to see Thestrals. However, Harry was present at Cedric’s moment of death though he didn’t actually see it, then he sees and has to return Cedric’s body. That should be enough for Harry to have “seen death”.

Neville and Luna were also able to see the Thestrals, though it’s unclear whether they actually witnessed the moment their loved ones died. We know Luna’s mother died in an explosion when she was nine, but we don’t know if Luna actually saw her mother die - we just know she described it as terrible, which doesn’t necessarily mean she saw it. It could just as easily mean Luna thought an explosion was an awful way to die, or simply that it was terrible her mother had died. There’s even less information about how Neville’s grandfather died, and whether Neville had actually seen the moment of his death.