Start with the damage done by cheaply-paid lightly-researched copypasta SEO spam junk pages to the trustworthiness of info returned by the typical Google search result. There are no shortage of people out there - or on HN - who know better than to trust most random webpages.

Now automate that to 10x, 100x, 1000x the volume.

Then throw in fake images and video too.

I believe they are referring to an exponential increase in noise alongside a relatively linear progression of signal.