Yes, the price increase is larger than the reduction in supply. Nobody wants to reduce their energy use by 6.5%, so everyone is trying to snatch up as much of the shrunk energy pie as possible, bidding up the prices in the process.

But there's a limit to the price war. At some point, a factory making widgets is going to look at price increases that make the cost of producing a widget greater than its expected sale price, at which point they'll probably stop their production lines, unless they feel like setting money on fire to keep them running. Those factory closures then provide for the necessary reduction in energy use. But everyone else still needs to pay the higher prices.

Fortunately, higher energy prices also incentivize increasing supply from alternative sources, so the EU probably won't have to reduce energy use by the full 6.5%. And as long as people setting money on fire are sufficiently rare, it will be energy-intensive, low-profitabilty users who give up first, so the economy likely won't shrink by 6.5% either. But a larger fraction will be captured by energy suppliers as profit.