Checking a few msds from the same supplier, I see they say that silica sand is not classified (hence the references you mention?) while silica flour and diatomaceous earth are classified as Xn and/or the CLP equivalent.

Depends what you mean by "silica dust"! Silca can be a natural mineral product in which case it most likely contains a significant percentage of the crystalline form (may be listed as quartz) of which the respirable fraction is a hazardous material and also classified as dangerous for supply. You'll find respirable crsytalline silica also has a WEL and is listed as a suspect carcinogen. However it can also be a manufactured product - for example amorphous or precipitated silica - which is not classified. So silica which contains a low % of quartz may not be classified, solid quartz crystals may not be classified but silica dust containing 90% respirable quartz is most certainly classified. Not so simples!