Can't they do destructive testing of a part of the fossil using some isotope with a longer half life than carbon 14? It's described as being in a field of bones sticking out of sand so it's the only possible way it could have been accurately dated - no surrounding sedimentary rock formation to do additional testing upon.
"Groucutt and the rest of the team used a number of dating methods to confirm the likely age of the Al Wusta finger. For the finger itself and the tooth of an ancient hippopotamus found nearby, they applied U-series dating. Like radiocarbon dating, the method works by looking at radioactive decay in the preserved materials. The age of the sediments around the bones was calculated using optically stimulated luminescence—a technique that reveals the last time rocks and sand were exposed to sunlight."
A nearby hippo bone was also tested. It seems rather unlikely that a human bone and a hippo bone of similar age would have been stumbled across somewhere else compared to this site which appears to be a dried up body of water.
