So, again, what are they trying to argue here? The only environmentally responsible option is to leave Gaza destroyed?

From the study itself (4. Discussion & 5. Concluding remarks), this is not what I got. On the contrary, it seems to me like they try to make some calculations/estimations/evaluations so that this is something that takes place.

The point of this study, to my understanding, is to calculates some of the aftermath of this ongoing genocide, concerning debris, carbon emissions, time frames, and implications for rebuilding. No?

Is this supposed to sway eco conscious genocide supporters?

This is not the way I understood this article. It seems to me it is one more argument for the case of continuous ethnic cleansing for so many decades.

If we agree that

most frequently, however, the aim of ethnic cleansing is to expel the despised ethnic group through either indirect coercion or direct force, and to ensure that return is impossible,

then by bombing all buildings, homes and infrastructure, they force people to go somewhere else and in the same time restrict them from coming back, since there is nothing to come back to.

So in this study, they actually measure the not coming back for decades part

Edit: I did several edits. I stop now.