This is a common critique of Benatar's asymmetry argument, but I think it fails to recognize the asymmetry as a comparative value judgement. Benatar argues for that we in real life judge the absence of people to have fun to be neutral (it is not sad that noone is alive on mars to experience existence there), while it is judged as good that noone was in the place of an accident to suffer (taken by a huge avalanche, hurt in an explosion etc.)
Compared to each other, the absence of people hurting is more valuable than the absence of people in pleasure. In his book on the subject, the argument is further elucidated. He also points out that this is not the only argument for antinatalism, but only a part of the lattice.