Further...a lot of the "Gardner made it all up" stuff comes from Aidan Kelly, who is himself not a very reliable source.
The thing is, Kelly is not the first, nor only, researcher to come to this conclusion. I heard all of this years before Kelly published, from the man who brought the Alexandrian tradition to the US in the sixties. My own background in Hinduism also made it clear what some of Gardner's sources were.
Whether it's "whole cloth" or "influenced by over a dozen different religions, compiled into an eclectic, modern tradition passed off as ancient" is really a matter of semantics, imho.
It does seem that Anderson had an influence from Appalachian folk magic, satanism* and hoodoo in his early versions of Feri (not sure if he was calling it Feri yet at that point), but he did bring in A LOT of stuff from Gardner when he discovered Gardner's books. Or that's what I recall Anderson saying in interviews.
*I don't say that to imply that Feri or the traditions derived from it are "satanic", only that in the research I've seen, it seemed pretty clear that Anderson took inspiration from folk traditions like, "Go into the graveyard, naked at midnight, and ask the Devil for magical powers." and similar stuff. It also seems like Anderson's presentation of what Feri was/is changed significantly over his lifetime.
And, well, there is the whole initiation story, which is exactly like Gardner's, IIRC.
While many people seem to feel drawn to Feri, I have to say that a tradition that combines Hawaiian, African, Celtic and Middle Eastern ideas, practices and symbology has to be modern. In my experience people didn't used to claim Feri was pre-Gardnerian. I only heard people start claiming that after Kelly was published and they didn't like what he had to say.