I think the labelling of it should be atmospheric carbon...

I think the labelling of it should be atmospheric carbon neutral. From the time the methane goes through the gate, the production of hydrogen and graphite produces no atmospheric carbon.

The definition is it carbon negative, neutral or positive only comes down to the feedstock only. Natural gas has extraction and fugitive emissions while bio methane from sewerage would normally be flared, therefore using it to produce hydrogen and graphite could be considered atmospheric carbon neutral process.

Realistically, the greatest impact on greenhouse gases at least in the short term is using natural gas, by reducing smr and the significant carbon this process produces.