And in addition to the implicit proof-of-resources of forcing attackers to run a bunch of Chrome slaves, there' are also explicit POW/S challenges in the code,

And in addition to the implicit proof-of-resources of forcing attackers to run a bunch of Chrome slaves, there' are also explicit POW/S challenges in the code, according to the article. It's quite an old idea [1], to add a cost which is trivial for users but a significant overhead for spammers [1] https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash