I read that as when Agfa used hydroxide in the preparation, they were able to use high quality hydroxide. Poorer quality hydroxides would certainly contain carbonate. And hydroxide that sits around with exposure to air will sequester carbonate. So it could be both a manufacturing and storage issue when compared to earlier sources of hydroxide.
So I take their statement as one that means they were able to manufacture the hydroxide with very little carbonate content - not that carbonate was used at any point in the process or from any other ingredients. I don't think that statement has anything to do with preparation from free base.
Perhaps they were using sealed reaction vessels and perhaps they had nitrogen atmospheres to protect the hydroxide solution from atmospheric carbon dioxide. It would denote a couple steps up in manufacturing technology from years previous.