Well, this looks like a section of kerb,

http://www.themodernantiquarian.com/post/16325

so natural or not it was definitely enhanced to look like a passage tomb at some point. It seems a bit high up for a barrow doesn't it? There are a few 'blind' cairns around so it being just a pile of gravel doesn't mean it's natural. The feeling back in the 1800s was that if there was no chamber or burial then it wasn't worth it. Why build such a thing for no explainable purpose?

I'm not sure it matters if it is a natural mound (which seems unlikely to me at least), the kerb and four poster obviously give the site some credentials.