What's concerning is that the post author seems not to see the problem with trying to sit on both sides of the fence at once. As others have said, the way this was done is likely to be against numerous laws in most major jurisdictions. If you wish to do this as a PoC then simply put a notice up on the page that initiates it and use dummy "competitor" content, so you've got some semblance of user content/transparency…
As a workaround, I recommend using separate Firefox containers for big sites, as the big sites are the main attack surface of a lot of people. I.e. Firefox containers for Google, Facebook, Microsoft. This attack would be stopped by using a Google container as the back button will not work once you step out of the container to go to the result page.
Sure, it won't help you on a targeted attack, but will help a lot with this kind of drag-net attack.