This is odd considering Prolific makes users input a bunch of demographic info (age, sex, location, education, ethnicity/race, etc) so that researchers can target specific demographics for their surveys. Did the researchers just assume that Prolific would deliver something balanced on it's own?
I think this is actually the right approach. Some 'balancing' algorithm would have to make assumptions about what's normal, then then the excluded participants wouldn't even show up in the research data.