I'd heard about this second-hand at Kalamazoo four years ago, where my topic for the ON FAIRY STORIES roundtable was Tolkien's knowledge of his fellow fantasists (e.g., MacDonald, Morris, Dunsany, Eddison, Carroll, et al.). Now I'm able to confirm this from a second source: Arne Zettersten's J. R. R. TOLKIEN'S DOUBLE WORLD AND CREATIVE PROCESS (a.k.a. MY FRIEND RONALD). In one of the many passing mentions of some topic which Zettersten says he discussed with Tolkien at one point or another, A.Z. mentions sending Tolkien a newspaper article which argued that JRRT had been influenced by Cabell:
"Tolkien wrote back to me and denied forcefully that this was true. The next time we met he took up the matter again and maintained that he knew Cabell well, but that he had read only one of this books and that it was 'quite boring'." [Zettersten, p. 199]
So, looks like we can take it fairly good evidence that (a) Tolkien was aware of Cabell's work and (b) didn't think much of him. I've heard the book in question was Cabell's JURGEN , certainly his most famous work, and one I wd expect Tolkien to find thoroughly uncongenial.
I do find myself wondering if Wilson and Cabell were among those Tolkien had in mind when he wrote in the Foreword to the LotR's second edition: Some who have read the book, or at any rate have reviewed it, have found it boring, absurd, or contemptible; and I have no cause to complain, since I have similar opinions of their works, or of the kinds of writings that they evidently prefer. Ouch. Especially in the suggestion that at least some negative reviewers hadn't necessarily bothered to read the book they were criticizing.
No way to know for certain, I suppose. But interesting to speculate.
P.S.: I shd probably add that my own opinion of Cabell is considerably higher than was Tolkien's: while I haven't read anything like his full works, I do recommend first and foremost JURGEN; anyone who enjoys that might want to press on to read whichever of the following he or she comes across, in no particular order: FIGURES OF EARTH, SOMETHING ABOUT EVE, THE CREAM OF THE JEST, THE SILVER STALLION, and perhaps THE HIGH PLACE. All these were published in the decade from 1917 to 1927; of the books he wrote earlier, I've only read one, which I found underwhelming; of the ones I've read from his final three decades, all combined triviality, self-indulgence, and a real underlying nastiness (the latter already all too present in THE HIGH PLACE).
Maybe at some point I'll be able to revive my 'Classics of Fantasy' column; I'd planned to do Cabell's JURGEN in the next three or four titles I got to (after Howard and Vance). We'll see.