The relative poverty rate is a valid metric, but the Guardian isn't using it right. The fact that they steer clear from even stating that it's only relative poverty being reported shows that they're not interested in actually informing their readership. But we already knew that.
But what's the definition of 'poverty' in that metric? If it's that you don't own a Ferrari then it's worthless in reality. If it's that you can't afford to pay for basic utilities and food then it's pretty damn important!