wonder what the unintended consequences of that much jellyfish food being made available is

This should be a stub wing plane shape with the prop in front and just drive through the jellyfish. faster, far less power needed.
It can go far deeper too as many more below.
This is going to become a much larger problem as the polluted seas warm which can't hold as much O2, holding more CO2 driving away fish, etc, jellyfish thrive. Fish, not so much..

don't they release their eggs when under attack, kind of a survival tactic,

So many times in the past, man has tried to solve a problem in Nature that has backfired! It struck me that jellyfish might be organisms that reproduce themselves from the parts like starfish and Korea might end up with millions of jellyfish where they once had thousands...Tim(e) will tell!

Why not eat the jellyfish? It seems a great waste to simply shred a good food source. I have seen a few articles that suggest they are a great food source and have been eaten in China for a "1000 years". Apparently a cafe in Washington DC, Jackey Cafe, serves a jellyfish dish. Try a simple Google search and then wonder why we would build machines to simply shred this resource!!!

No mention of energy source and endurance for these robots?

How do they prevent by-catch? Just by the strength of the draw toward the prop? How would it deal with something less shreddable, like a plastic bag? The swarm stuff seems like technology for its own sake - surely a three meter piece of plastic rod would have kept the three "swarmers" apart even more accurately and at almost no cost?

How much methane do jellyfish produce when decomposing?

Yeah, it has been many times that man attempts to deal with consequences, instead of reasons. All proved unsuccessful. Jellyfish did not multiply for no reason, did it? I believe it is a wrong effort, in spite of good intentions. The only sure way to get rid of too many jellyfish is to withdraw the reasons that favored their rapid reproduction. If I remember correctly, the increase of water temperature, which hampers other sea creatures life and reproduction, helps jellyfish... During the last 5-6 years spearos have observed the presence of several Aegean sea fishes in the Black sea where they have never been seen before! We believe they are looking for cooler waters...

A bounty on jellyfish might work, the carcass might be used for fertilizer or cat food .