Top comment from the Hacker News discussion:

The title is false. It seems to derive from the following paragraph: “The results were alarming, with 15% of the top 100 subreddits found to have content that was likely posted by bots or corporate trolls, specifically aimed at promoting certain companies or organizations.”

It's not 15% of content. Nowhere even close. It's 15% of top subreddits that might contain just a single post considered "likely" (no actual verification) to be from what they call a corporate "troll".

In other words, it's a completely meaningless statistic.