I can definitely get behind some of the solutions they propose! But it is important to give a truthful rationale for these. Otherwise this is a slippery slope, like lying to your children about medicine because they will get better if they take it.
I stand by my assertion that the methodology they picked ("pick a subset of civilization, make a prediction as if everything was it") is fallacious. Imagine a sustainable, closed-loop civilization, satisfying all the goals you can dream of. It is inevitable that not every part of the system will be balanced when looked at in isolation. If you are allowed to cherry-pick a subset of that civilization and scale it arbitrarily, you are guaranteed to arrive at the same unreasonable conclusion as they give us.
To make it less abstract, let us consider nature itself. There are siberian tigers which eat 9 kg of food a day. This is a lot! If every animal were to eat that much, all animals would quickly starve. Clearly, nature is not sustainable!
(And there will be plenty of incentives to come up with cheaper steaks or steak alternatives.)
Why so?
Life needs energy, we can harness energy better to produce engineered food and desalt water.
It's just an easy to understand illustration of how our current civilization is largely resting on unsustainable foundations. Finding solutions to the engineering and societal challenges that you mention seems to be the goal of the project.