It is designed by researchers who have been studying this for a while. At least for the near term the upkeep is worth it because people who previously no had prospects can now grow enough food to live on. You have to keep it up, but keeping it up means you can eat so the constant upkeep shouldn't be a problem so long as you are healthy. (culture or something is needed to deal with those who because of age or injury are not able to keep it up)

The real question is if in 50 years - as the current people use the fact that they have food and some extra invest in education for their kids, and so the kids move away to better city jobs. Those that remain likely love agriculture, but will be looking for ways use tractors to do the hard jobs. This doesn't seem to scale to large tractor operations. Those researchers thus need to look ahead to what follows this in 50 years.

This requires constant upkeep. The goal is to structure it so the local community is incentivized to invest that upkeep, but it's not just gonna happen all by itself. The goal is indeed a self-sustaining ecosystem, but one with human participation. If you want more detail, a good keyword is "Farmed Managed Natural Regeneration"

That keyword also makes it pretty clear it's more than a shovel and some seeds :)

And, fwiw, your request made me look at the thegreatgreenwall.org, and... good god. It is one of the lowest-information sites I've seen in a while. You could spend hours on there and learn nothing. https://thegreatgreenwall.org/science-and-the-ggw is as far as I can tell the only concrete part of this piece.

A much better starting point if you care about a bit more than feel-good vibing is https://www.unccd.int/our-work/ggwi

And that site makes it again abundantly clear that this is a very large scale project. The difference from the Qattara Sea project is that it actually managed to gin up multinational corporation, and that it takes a long term lens (as opposed to "IDK, let's flood this, rest's gonna work out" of the various seaflooding projects). And, most importantly, because it integrates the local communities in the project.

The last part matters because any such project is far from "fire and forget", and you need a strong local stake in any such project for it to succeed.

The citation is the video linked to that you’re commenting on. This isn’t tree planting greenwashing. This is engineered and planned, but it involves local people tending the land and turning it into continuously productive soil for profit. Greenwashing tree planting fails because it’s untended unplanned work. In this video they go through all the steps they take, including analyzing contours and building the water retention berms along them, the phasing of the work from basic planting to trees with crops interspersed etc. It’s really worth watching it.