That would be a very difficult experiment to design. Whatever process one used to isolate heavy water would invariably affect the concentration of trace amounts

That would be a very difficult experiment to design. Whatever process one used to isolate heavy water would invariably affect the concentration of trace amounts of other molecules. While the two samples might taste subtly different, the difference might be due to some mineral picked up (or lost) in the separation process. I wonder how rigorously they controlled for this? Would you run a spectroscopic analysis on both samples to ensure they were identical but for the DO2? And, even then, one would have to wonder whether the density of the liquid may be having some subtle effect on the manner in which contaminants—-even if identical in concentration in both samples—-are being uptook into the taste nerves.