With other assumptions, they would arrive at 0 intelligent civilisations in the galaxy. That would make the rest of the paper void. :-) Nobody knows how life i

With other assumptions, they would arrive at 0 intelligent civilisations in the galaxy. That would make the rest of the paper void. :-) Nobody knows how life is distributed in the galaxy. Assuming that we are not special (the Copernican Principle) is a valid prerequisite. As it neither can be proved or disproved, it's not unserious to just state what the unproven assumptions are.