Garner's advice, to paraphrase—his entry on who/whom is, unsurprisingly, long—amounts to generally understanding and following the rules, but using whichever reads or sounds better if rule-following results in awkwardness, as it certainly sometimes does (he has examples). In this case I'd judge that either works, though the more formal-register the writing the more this use of "who" would stand out.
This being a headline in the Economist—and considering that, apparently, at least some found this use of "who" distracting, while "whom" would have been at least as easy to parse and probably wouldn't have bothered anyone—it's probably fair to call it a small error.
Garner also notes that predictions that "whom" is on its way out are, at this point, part of a long tradition, and do seem to capture the trend but do not yet describe current standard usage outside the fairly informal or colloquial. IOW for the best hope of communicating well at higher-formality registers, especially in writing, do continue (mostly) following the standard rules, in this case, avoiding it only when "correct" use is distracting or strange to a modern reader.
This being a headline in the Economist—and considering that, apparently, at least some found this use of "who" distracting, while "whom" would have been at least as easy to parse and probably wouldn't have bothered anyone—it's probably fair to call it a small error.
Garner also notes that predictions that "whom" is on its way out are, at this point, part of a long tradition, and do seem to capture the trend but do not yet describe current standard usage outside the fairly informal or colloquial. IOW for the best hope of communicating well at higher-formality registers, especially in writing, do continue (mostly) following the standard rules, in this case, avoiding it only when "correct" use is distracting or strange to a modern reader.