We sort of live in a finite universe in terms of matter and energy right? Conservation laws seem to imply that. So at a large enough scale life would have to be zero sum. I guess what people may actually be arguing about is at what scale the finite aspects of the universe actually matter?
On an anecdotal level, while thinking that everything is zero sum all the time probably isn't good, I find that the majority of the people I interact find it very difficult to imagine that anything is "zero sum." Since they associate "zero sum" with mean things like harsh competition. It seems like that's its own sort of "bias."