Very interesting article, had not read it before.
It does bring up an interesting thought about another Tolkien character that has always defied classification, even despite being given one. I speak of course of Ungoliant, Demon of the Void. That title is very vague, and implies that the spirit that formed into Ungoliant once it arrived on Arda was a being wholly independent of Eru's plan and creation. I think this hints at the possibility that Eru was not the sole inhabitant of the Void, as we seem to think, because why else would he create something like Ungoliant? Or was she a rogue Ainu that snuck in before the others?
I watched a video on YouTube where someone undertakes to theorize on the origin and nature of the "nameless horrors that gnawed at the roots of the world", and the fact that they were older than even Sauron.
The supposition is that these creatures/beings/spirits were the unfortunate result of the discord created by Melkor when he defiantly tried to weave his own theme into the Ainulindale. These misshapen things fled to the newly formed Arda and hid there, coalescing into abominations, like the Watcher in the Water, and the other beasties Gandalf glimpsed when he fell into the Underdeeps in his fight with Durin's Bane.
At any rate, all this to say that if one takes that school of thought, Tom is not really alone as something from the outside that doesn't really fit. He could be the thematic opposite of something like Ungoliant, who was also not part of the grand design but ended a permanent resident.
Next question... who or what is Goldberry?