Not all wetlands are methane-producing. I’d imagine that wetlands formed where ice has just retreated first have to sequester a substantial amount of carbon from the atmosphere, before they could release it back again.
Of course there are relatively small areas of greenland that have been green for a long time, so may have accumulated peat, but to imply the ex-ice-sheet will became a big methane source, just because this is an issue in other regions of the arctic (russia, canada) , seems to extrapolate too far.