FRANKFURT—A spike in euro zone inflation to above the European Central Bank’s target is a temporary “hump”, the ECB says—dusting off a metaphor used a decade ago by its then-president Jean-Claude Trichet. In 2011, the ECB hiked interest rates twice in four months, with Trichet yielding to German pressure despite believing that a commodity-fuelled rise in prices would prove short-lived. That is now acknowledged even by the ECB itself as a policy mistake that exacerbated the euro zone’s debt crisis. Having learned the hard way, current ECB policymakers say they will not tighten monetary policy until inflation is stably at the bank’s 2 percent target, which they don’t expect to happen for more than two years. “The inflation outlook remains characterised by a hump in 2021 followed by more moderate rates in 2022 and 2023,” the ECB said last week, blaming recent price rises on more expensive raw materials and …