Sanders is right, or at least he's right in saying 10% is a lot closer to the truth than 5%. The latter figure ignores people who have given up trying to find work for one reason or another. Another statistic that's relevant to this discussion is work force participation, which is at its lowest point in about half a century -- that's the percentage of people of working age who are employed or seeking employment. And of course people with part-time jobs skew the numbers, too.
On the other hand, if the idea is to compare the current situation to previous years, you have to use the same standards, which is where the 5.1% comes into play.