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.