The scientific paper stated that the rhino's age came out to 709000 +/- 68000 years. The number was consistent with other measurements made of the age of the depositional layer.

Before releasing a claim like this, I would have to believe they were extremely careful in the testing and interpretation. If they have this wrong, it could be a career killer.

The article links to a description, i did some hunting but was unable to find any hard number for a margin of error.

Electronic spin resonance: ESR, which measures trapped electrons using magnetic fields, is related to magnetic resonance imaging, the medical technique that allows doctors to look for tumors or peek inside your creaking knee. Because ESR essentially tracks the activity — the “spin” — of the electrons without freeing them, the sample can be subjected to repeated dating attempts. ESR also has a longer range — some researchers claim up to 1 million years — but it’s more complicated than other trapped charge methods, leaving it more susceptible to error.