all of those languages implement recursion in one or another way

Yeah—Python and English are both recursive, so that doesn’t account for why the brain processes them differently. But they need to figure out what other feature does account for it—ideally by finding a pair of (probably artificial) languages that differ only in the exact feature which triggers the language network. Then they can figure out how that feature relates to recursion or any other mental abilities that might have co-evolved with language.