If the tomb has been opened it gives them an out if the DNA of the purported Alexander III does not match that of Nicholas II. If it does match, no problem. If not, it does not necessarily mean that Nicholas II's results are invalidated. It could equally mean that the tomb (and other nearby tombs) were opened at some time after the interment, and the remains mixed up. That is quite possible, as I know of a family mausoleum in Poland where the tombs were rifled by Russian soldiers in WWII and the skeletal remains were put back into coffins by guesswork.