I was quoting directly from my copy of the Silmarillion. But I don't agree that that later paragraph invalidates the premise that the lore of LoTR has its roots

I was quoting directly from my copy of the Silmarillion. But I don't agree that that later paragraph invalidates the premise that the lore of LoTR has its roots in an attempt to create English mythology. This letter was written after LoTR was written (but not yet published), and it was inspired and written under the very effort that he later give up. That he didn't end up publishing the entire mythos doesn't mean that it's not a valid lens under which to interpret his work. Indeed, it's a necessary lens.