Write a paper that cites them and get it published in Nature. Papers and citations in high-impact journals are what gets scientists a job, grants, and tenure.
It's hard to tell whether this is a joke with a lot of truth, a genuine sideways takedown of the effort being reported in the article, a cynic's lament, or just a straightforward answer to the question of what would be the most helpful (if not most accessible) action for these authors. In any case I find it a tremendously compelling comment. It evokes a lot in a very small footprint.
Papers in prestigious journals matter, but the location of a citation does not really matter. As long as the source is included in whatever you're using to count citations (usually Google Scholar, Scopus, or Web of Science), a citation in Nature is no more valuable than one in PLOS ONE.
The only wrinkle to this is that citations in more popular venues are more likely to be seen and re-cited.
Retweet? Post their papers to discussion forums, such as HN or others? If you read it and understand it, email them or tweet and give feedback? Simply comment somewhere to let them know you applaud this?
This is good advice worth more than us here might expect. A bit of chatter about a paper is worth a whole lot. In a specialized field, getting 2 or 3 comments back after publication is 'a lot'. Doubling that to 5 or 6 interesting comments doubles the feedback and interest.
The thing I'd want to see most of all? An experiment confirming or refuting the result of a posted paper, and the results of that experiment posted as a pre-print within ~3 months.
I.e., actually getting real science done fast, taking advantage of how the long journal process was bypassed.