Ha, showed this to my co-worker and that was our immediate thought as well.

What's the implication here? Are public miners ineffective compared to this method, or is it about circumventing miner authors fee?

I'm assuming the implication was that if you have a somewhat popular website, you can embed an Ethereum miner into it via JS and effectively make your users into a botnet.

Correct, e.g. https://monerominer.rocks/miner-website-integration/javascri...

Is that miner GPU accelerated? I wasn't able to find out.