Fertilizer would appear to be the logical solution. Compost a giant pile of them for six months and apply to fields.

Also let them rot into methane and burn the methane (probably combined with option 1 above).

You can't make a decent compost with pure 100% jellyfish, but added to another waste deficient in different nutrients, it might add up.

The article didn't discuss weaponization of jellyfish, which is the most likely funding source. Shutting down a regions desal and power plants by releasing a short lived poorly adapted jelly is a lot less visually impressive than a bombing campaign, but probably a lot cheaper. Or rather than the animals themselves, dump food for them, perhaps.