I can see where he's coming from.
I have no idea what "China clays" are made from. I know what my slugged ABS chips are made from - ABS plastic. And they sound very similar. Both are about as "slippery" as the other (the China clays are slightly less slippery, which is explained by the pressed-in texture like the stitch or crown patterns). The only obvious difference - apart from the moulding - is that ABS plastic is a lot harder, wherea China clays can be marked with a fingernail.