They’re certainly advertising as such. In practice this is how it works: GreenGeeks tells the Bonneville Environmental Foundation (BEF) how much electricity they use. The foundation calculates how much green energy they’ll need to use to make up for it. GreenGeeks purchases that amount in renewable energy.
So it’s not like their servers use less energy than standard hosting. However, they are certainly committed to making up for what they consume, and using as much green energy as possible.