I suspect there's always a human checking these results. If NPM straight out rejects an update due to suspected malware, they might end up rejecting correct updates as well. If they grant some "safe" patterns a special pass, they might get exploited.

So I think this only works if you have security scanners that are well-maintained and kept in secret. NPM folks could of course co-operate with some security companies to have a first stab with the releases before they are put to public access. At some point some parties might start want to have monetary compensation for such an arragnement, though.