Except you can make hypotheses about what you could expect to see as negative effects, feed a population of volunteers (or animals) the food, compare them to control population and see if the hypotheses can be accepted or rejected? Complaining about proving a negative or proving something is safe as a knee-jerk reaction is silly. Think about what happened with all the regulations around BPA in plastics food containers. The industry worked around it by substituting BPS and BPF. Demending those alternatives should have been proven safe first is entirely reasonable.

I will admit that Greenpeace will probably just shift goal posts continually with "it's not proven safe enough" while simultaneously never putting in writing what the bar for safety should be..