Did Lester play a part in shaping fantasy fiction? Of course. Did he "invent fantasy fiction as we know it"? Come on. If you trace things back arguably something like "Der Ring des Nibelungen" is the invention of the fantasy genre, but is even that it!? It's perhaps not even a modern opera since it's from 1869. If you had to point to only one author it would certainly be Tolkien, but he didn't write in a vacuum and was certainly inspired by others as well.
So can anyone here speculate as to why Slate and Dan Sinykin decided to publish this piece, they make a completely offside mention of white supremacy and sexism towards the end, so I'm drawn to suspect this is just another piece trying to rewrite history to make it seem like modern and less "problematic" authors invented genres we all cherish.
Before Del Rey fantasy was fringe and hard to find (apart from Tolkien, obvs.) After Del Rey it was mainstream and a huge commercial success.
There was a post-Tolkien market slot and Del Rey filled it.
It was more innovation than invention.
No, never mind. I can't fake genre-ignorance about Slate the way they can about fantasy.
The goal isn't to rewrite history, it's to draw rage-clicks and invite proud corrections, in other words waste our time, for their advertisers.
That doesn't work in this case, because the article calls out Lester's early work as 'absurdly' sexist and claims that the standard playbook for fantasy required a male central character. He seems plenty problematic to a modern sensibility. If anything, the opposite seems like a more plausible aim - to claim that Fantasy as a genre is tainted by its sexist origins.
Incidentally, the US Ballantine edition of TLoTR had massive publicity from Tolkien himself, in response to an earlier unauthorised publication in the US by Ace Books, for which Tolkien didn't receive any royalties. Tolkien was uncharacteristically active in helping to suppress the unauthorised edition, leading to a major grass-roots campaign and useful publicity in US universities and the wider counter-culture.