I think this is the one anyway. Seems like it points to "cognitive inflexibility" rather than the interpreted version of "performs poorly," so I'm not sure if this is just the journalist stirring up shit with an inflammatory headline (oh my word how unheard of) or there's another study I couldn't find.
I agree the journalist and arguably the researchers themselves are representing these results poorly.
Participants across the studies were evaluated on their likely hood to die for the in-group or kill the outgroup. The groups in question appear to be nationality. So "Extreme ideology" is better read as "extreme nationalism" imho.
The finding is, as you said, more rigid cognition. Less lateral thinking. Not "extremists are dumb", which is a feel good line that undermines the threat of certain extremists.
I really want to give the benefit of the doubt and assume there was a followup study with the things the article brings into the discussion: liberal vs conservative thought patterns, or cognitive inflexibility correlating to poor analytical ability.
But I have no idea and wasn't invested enough to do more than google "zmigrod extremism study" honestly.
because it's bullshit lol. either that, or it's a terrible news outlet. all too often journalists think they can get away with bullshitting by simply failing to link to any source. people should just auto-reject any news posts that lack sources tbh.