What are fecal implents?

Sorry, typo. Should be fecal implants or stool transplants.

Few people do, and I doubt that it will catch on; spirulina can also be grown on runoff fertilizer, which will probably sound more appealing to most people.

Sounding appealing is a question of marketing. Plenty of people prefer organic food that grown with feces of animals over food grown with "chemical" fertilizer. They even pay more money for the product.

I also think you underrate the cost of fertilizer for some poor biohacker in Neirobi who has plenty of access to empty bottles. Human urine should also be pretty cheap to buy in third world megacities.

Access to cheap natural gas and oil is also central for the current way of doing agriculture. Without having access to those resources for cheap prices resource reuse might be a bigger deal.