First we have to compare two types of flaring, just releasing the gas unburned where the methane goes directly into the atmosphere and acts as a powerful greenhouse gas (rarely done these days...intentionally, anyway), and flaring and burning it where the methane is ignited into an open flame that mostly converts it to CO2, a longer-lasting but much less powerful greenhouse gas. Previously the energy wasn't used at all, just wasted, but that was an energy waste issue rather than an emissions issue.
Burning the gas in a turbine converts it to CO2 more efficiently than an open flame, so the environmental improvement over the first scenario is obvious, compared to the second the improvement isn't as huge but still significant. I'd guess the 63% CO2-equivalent emissions is the difference between the total effective GHG output of running the gas through a turbine vs. burning it in an open flame, since some of the methane can still escape that way.