I've had this earmarked for a while and have finally listened to it (I know, I know, it's a video... but I really wasn't watching it).
Flieger's argument--which is well-articulated and well-argued--seems to me to boil down to "Tolkien never really wrote about Eärendil
directly and that's intentional: he's deliberately crafting a lacuna in his mythology that mimics and echoes the lacunae in real world mythologies."
That is my reduction--I far recommend that people watch the video, since it's more detailed than that (and more persuasive).
My gut instinct (and it seems John Rateliff
agrees) is that I'm not persuaded. It's impossible to argue the non-existence of something, but I think it's worth noting that there isn't a shred of commentary that Tolkien ever made to suggest this was his project--and while Tolkien could very much be coy about things and didn't necessarily always write the strict, literal truth, he
was willing to talk about his craft to a considerable extent. If it were some private intention, it's certainly possible that he'd keep it under his hat, but there'd be more to support it: there'd at least be marginalia somewhere: Tolkien made calendars and schemas and timelines and notes upon notes.
That said, if Flieger wants to tweak her argument to suggest that Tolkien eventually (say by the late 1950s) came to realise that Eärendil was starting to resemble one of the classic lacunae of ancient literature and decided to lean into it, I could see that as more plausible. Until this point (i.e. until after
The Lord of the Rings had been published), Tolkien expended considerable effort on
The Silmarillion with every apparent intention of finishing it--and indeed, the whole delay on the publication of the LotR was that he wanted it as a combo!
The trouble for Eärendil in both the 1930s and 1950s--because he actually DID write about him in the 1920s in the
Qenta Noldorinwa--is twofold: having never written a full version in the
Lost Tales era, Tolkien had no base text to borrow and niggle over, as he did Tuor, Beren, and Túrin, and that, coming at the end, Tolkien never quite made it there--the problem with constantly starting things over from scratch is that the end may never be reached!
But, after the LotR was published, Tolkien takes a noticable turn towards a meta-view of his work. The whole attempt to revise the astronomy is part and parcel of this, as is the growing idea that the old conception of a flat Earth must reflect the transmission through Mannish hands. So there is ample evidence at THIS point that Tolkien might have been taking a more outside-the-story perspective. His silence about it, in the last 15 years or so of his life, also makes a lot more sense, because he had committed to writing the Silm, but was never making any more progress on it. Of course, going back to the days of
The Hobbit (or back to his academic works in the 1920s), there's ample evidence of Tolkien overpromising things to his publishers before being distracted. So, of course he wouldn't say he was deliberately putting in a giant lacuna. At the same time, it is the sort of realisation that might have helped salve his conscience a bit over having failing to really complete the project.