I promise this isn't just a well askually but anaerobic digestion (what is being discussed in the article) and composting are different.

Composting requires oxygen. It produces carbon dioxide. It produces heat. Compost has less restrictions for use as a fertilizer than digested mature. I'm not from the US so I don't want to assume it's the same across all states but in Canada you can sell it to the public.

Anaerobic Digesters do not require oxygen. It produces carbon dioxide and methane. It requires heat input to occur at a reasonable rate. Use as a fertilizer is restricted because more pathogens survive. Hydrogen sulphide is produced so sometimes ferric chloride is added, and that reduces the amount of phosphorus available to plants.

One of the trade offs with digestion is that processing the digestate into something usable takes additional energy. If it's a single farm and they are not taking additional feedstock it's no more nutrients that they had before they started digesting. Chances are they can manage the nutrients on site and don't need to input energy.

If they are taking stuff from off-site, or if they expended their operation beyond what they can manage on site then they need to choose if they are going to transport it off-site as a liquid, thicken it into cake (mmmmm), or create a fertilizer product like pellets. Each option requires more energy than the last. They may be able to sell the pellets but typically have to give liquid or cake away for free or pay people to take it.