“The Good Soldier” by Ford Madox Ford: How can first line—“This is the saddest story I have ever heard”—be true for first-person protagonist?

I am halfway through this author’s masterpiece, whose famous first line (see above) is puzzling: How can the first-person narrator, who is one of the main characters, speak of the story as something that was told to him?

Looking for opinions online, I find that some writers simply ignore the issue, but that of those who address it, there are two approaches: 1. rationalizing it (making excuses for it) and 2. taking it as the first evidence of an unreliable narrator.

Rationalization
"This is the line written by John Dowell that opens the novel...The word choice in this quotation is very important. This is the saddest story that Dowell has ever heard. The word 'heard' implies that he has not lived the story or experienced the events, but merely 'heard' about them. His statement is accurate. Although Dowell was present in many of the scenes he described, his eyes were closed to the reality of what was occurring. He was so blind, ignorant, or naive, that the significance of the events can only be felt as he writes and reflects upon what has happened" (1).

Unreliable Narrator

" 'This is the saddest story I have ever heard.' What could be more simple and declaratory, a statement of such high plangency and enormous claim that the reader assumes it must be not just an impression, or even a powerful opinion, but a 'fact'? Yet it is one of the most misleading first sentences in all fiction. This isn't - it cannot be - apparent at first reading, though if you were to go back and reread that line after finishing the first chapter, you would instantly see the falsity, instantly feel the floorboard creak beneath your foot on that "heard". The narrator, an American called Dowell (he forgets to tell us his Christian name until nearly the end of the novel) has not 'heard' the story at all. It's a story in which he has actively - and passively - participated, been in up to his ears, eyes, neck, heart and guts. We're the ones 'hearing' it; he's the one telling it, despite this initial, hopeless attempt to deflect attention from his own presence and complicity. And if the second verb of the first sentence cannot be trusted, we must be prepared to treat every sentence with the same care and suspicion. We must prowl soft-footed through this text, alive for every board's moan and plaint” (2).

Are there any other possible explanations?

1. https://www.sparknotes.com/lit/goodsoldier/quotes/

2. https://www.theguardian.com/books/2008/jun/07/fiction.julianbarnes

3. Ford Madox Ford. The Good Soldier: A Tale of Passion [1915]. Edited with Introduction and Notes by Max Saunders. Oxford, Oxford University Press, 2012.