This [to put it midly] controversial German Historian claimed
recently that the Nazis carried out a successful test blast of a
'small' atomic bomb at a place called Thuringia near the end of the war.

Do we put this claim in the same category as UFO's, Bigfoot,
Princess Diana assassination conspiracy, Elvis sightings?

Or is there any credence to what he says?....His claim is testable:
if there where a nuke blast in Thurinigia, there should be radioactive
glassy deposits [trinitite] around there.

Personally, I give it as much credence as say: Hitler's Nazi cronies escaped to the planet Mars to establish a Nazi government in exile, living in an underground Mars base there to this very day with clones of Hitler & Mengele.
[Yes, there is actually a bottom-feeding website out there somewhere in cyberspace that makes that crazy claim]
BTW, I lost track of that very cheezy and nutty website; wouldn't mind
if someone digs it up.