Remember this is carbon originally sourced from the atmosphere by plants. So burning methane sourced from waste isn't actually introducing new carbon, it's just skipping its 20 year life as methane. So it is in fact a net positive over the natural course of the carbon cycle. This is in effect a true renewable as long as it's not fossil sourced natural gas we're burning. I don't think we can scale an entire energy economy off of the byproducts of waste, but there are applications where this makes sustainable sense.