It is literally the Haber-Bosch process, without question. The school actually calls it that in their own paper, https://cbsi-asabe.org/wp-content/uploads/2024/06/Harvesting..., because that is what it is.

I assume what confused the article is that ammonia plants often use natural gas, but that's purely as a convenient energy source and a source of hydrogen, so they cover both angles. But that isn't remotely a necessity.