Chances are, it's not a Thuringian. All we know is, it's dark, and a slate. Also, it has some different coloration that is not characteristic of a Thuringian, that's a fact.
Earth has plenty of slates to offer, a lot of these quarries have some homogeneous material that can be used as a hone.
Schwedenstein (it just might be one but interesting patterns in slate is nothing unique) stones aren't Thuringians, it's different area and a different quarry, same case with the mottled type of stones that are rarer than true Thuringians, but aren't the original Thuringians either, not that they are inferior in any way.
Schwedenstein stones have good cutting power, and Thuringians don't, that's a big difference. The ones I tested were finer than the classic Thuringians too, but that's not a fact as there are softer ones that are coarser and not of finishing grade, I hope to get one of these next year.
Also, any slate can have good cutting speed with or without slurry, or it can be slow. It's the nature of the material, not an indicator. Slurry color isn't an indicator either.
I seriously doubt it's a Thuringian unless proven otherwise. All we know is, it's a fine, probably, dark slate. The market is full of sellers with stones marketed as Thuringians, and in most cases they aren't. Someone assuming that a stone is a Thuringian doesn't make the stone a Thuringian. And proving that one is, that's a real challenge.