Rather than using the photovoltaics to directly generate electricity for the grid (via DC to AC conversion of course), they’re using it to make the digestion process “self-sustaining” — then turn methane into electricity in a fuel cell in order to produce hydrogen. Pelion on Ossa.
It isn't methane they're converting, but the larger and more obdurate molecules (note the statement "dark wastewater becomes clearer"). Methane isn't "dark".
So they're using energy from the photovoltaics and energy from the hydrocarbons to get more total energy than either can get when run as a standalone process, while simultaneously cleaning the water. This still sounds like a "good thing" to me.
As to "why hydrogen" instead of direct electricity?? I dunno. I kind of wondered that myself. It may have to do with what their "electrogenic bacteria" are capable of.