Though I can't still wrap my head around it:
1. Terraform needs to produce hydrogen using electrolysis. Turning hydrogen into natural gas requires more equipment and energy.
2. Simultaneously, we transform natural gas into hydrogen through natural gas reforming. Currently, hydrogen is vital for fertilizers and refineries. This also uses energy and a lot of equipment.
So, before we produce synthetic natural gas, displacing hydrogen produced by fossil fuels with electrolytes should be more economical.
On top of that, fertilizers such as ammonia are way easier to ship worldwide than natural gas.