I suspect there are a few explanations. Firstly it was a mistake...

I suspect there are a few explanations. Firstly it was a mistake and they meant to say "carbon negative". Secondly they couldn't claim it to be carbon negative because they have not connected either partly or fully to renewables to power the process. . Hazer did include an electrolyser to convert some of the hydrogen back to power. They've not reported anything about its operation to date. Thirdly the feedstock used is partly or fully run on natural gas rather than biomethane. Either way it would be nice to know because it was one of the big claims for a world first carbon negative hydrogen process to be cleaner than hydrogen produced by electrolysis.