There are a lot of different slate stones like that, it's hard to tell how good it is until you get a razor on it, perhaps get some pics of an edge against pictures of an edge from a known stone, etc. Even in the US there is a lot of slate and chert that is not novaculite but that will finish a razor. For various reasons (jasper comes to mind), it's either extremely hard and oddly shaped because it's not cut into a stone for honing because there are other users (jewelers) or there just aren't big enough pieces, etc.

There are also a lot of stones capable of finishing a razor that just don't do it that well. If the shave is like a coticule, then for a slate that's problematic, I'd say. It depends on the coticule, too, but it may be that the particle size in that stone is a bit large. Who knows?