Although methane is a short-lived greenhouse gas, once emitted into the atmosphere it causes a lingering warming effect long after the methane itself has gone.
If emissions from New Zealand’s livestock were held at current levels, warming caused by those emissions would increase for centuries (although at a gradually declining rate). While methane emissions from New Zealand's livestock may be short-lived in the atmosphere, they are by no means benign.
If the country’s desired climate change goal for livestock methane emissions was no additional contribution to warming, then these would need to reduce by some 20% by 2050.
Modelling alone cannot determine how methane should be treated when setting the country’s climate change targets. That is a policy matter. The objective of no additional contribution to warming beyond current levels is only one possibility.