The problem with the essay is that its author has fundamentally misread Tolkien and Howard.
On the contrary, I feel that the problem here is that you have misread his position.
For one thing, you are missing one extremely salient point: that both Tolkien and Howard differ from the nihilists Grin mentioned because there is genuine heroism, hope and exultation in their works.
Tolkien is full of loss, sorrow and decline, but it's also full of great men doing great things, be they humble Hobbits, or heroic Kings of Men. Howard's stories are full to the brim of similar examples.
THAT is what Leo is talking about which is missing in the stories of which he speaks. There are no heroes, no beacons of humanity to show that while the universe is a dark, cruel, unforgiving place, there are those who defy it with shining beacons of optimism, altruism, and bravery. How can anyone describe anything like that as Nihilistic? Surely if a work is nihilistic, then there would be no victories, no heroism, no hope?
There are no Frodos, Aragorns, Theodens, Balthuses, Solomon Kanes or Kulls in the works Leo is decrying. In short, there are no heroes in these particular works. You say The Lord of the Rings isn't a happy ending: true, but vastly preferable to the alternative. It is an ending I can happily call bittersweet: victory and peace at great cost. Nihilistic? Certainly not.
Your mention of Grin's dismissal of Wheel of Time, and your similar argument that Grin seeks a more "black versus white" approach, is also inaccurate, in my opinion. I read his argument as being that there should be more distinction between the shades of grey going on. Black, white and grey, not just lighter and darker shades of grey.
I'm actually not sure if my rebuttal was sufficient: doubtless someone like Brian would do a better and more articularted job of explaining how, no, Leo did not misread Tolkien or Howard. I'm convinced that the blog author has simply misread Leo's argument, not that he disagrees with his points. Either that, or he really thinks Tolkien's a nihilist...
Special mention should be given to Maurice's comment, which is the most profound misinterpretation of a Conan story since Unhappy "Conan threatened to rape Yasmina in the burning ruins of her palace" Anchovy:
I remember also thinking about characters like Conan being chivalrous until a friend of mine told me to actually read an original Howard, The Frost Giant's daughter, where he basically chases and kills a woman's family of giants just so he can rape her.
I'm impressed with how easily you colour that story by the choice omission of several extremely important details to give a completely fallacious and disingenuous impression of the plot in order to make Atali and her brothers out to be the helpless, innocent victims of a brutish Conan.
You miss out the extremely important detail that the woman in question was hypnotically compelling Conan to follow him to his death. She has done this to unnumbered men through the millennia, appearing to dying men on the battlefield, luring them to their doom through sexual manipulation and supernatural compulsion. Men may be bleeding to death and exhausted, but a "strange madness" forces them to walk leagues upon leagues far from civilization in pursuit of a mocking, taunting goddess. Said family of giants, by the way, were lying in wait in preparation to murder him - once again, as they have done to COUNTLESS others - and sacrifice his heart to their father-god.
The guy who wrote the article must have never read one of Howard's works.
Leo Grin is one of the most respected Robert E. Howard scholars out there. His journal, The Cimmerian, is highly regarded, and twice nominated for the World Fantasy Award. He most certainly has controversial opinions not all Howardists agree with - many Howardists disagree on multiple subjects - but to allege he has never read Howard's work, or that he has misread them, is simply preposterous.
Given your grotesque misreading/misrepresentation of "The Frost-Giant's Daughter," I have to question how on earth you came to such a conclusion - or even if you've read the story at all.
In fairness, the near-rape aspect of "The Frost-Giant's Daughter" is often misunderstood: however, the way he implies that Conan just raged into a peaceful giant's town and slaughtered an innocent girl's family as a counterpoint to Conan's chivalry is actually rather nauseating.