I actually agree with the use of the word "Gentrifying" here. The way I understood it, the author is implying that the internet land grab is being gentrified in the same way that many neighborhoods have been. The rich roll-in on land that was once considered worthless and take-over, displacing the original inhabitants, generally the poor.
Now In the context of SEO, when the web first started, the big scary corporations that had tens of thousands or even millions of dollars a month to spend on site optimization, only focused on specific areas of the internet. Insurance, Lawyers, general e-commerce, etc. There was a war for attention and land value in these sectors of the internet, and SEO were the soldiers of that war. But for the most part, that war stayed "over there". People that wanted to share recipes, hackers that wanted to share code snippets, families that wanted to share photos, and sites dedicated to niche hobbies all continued to live in the original frontier of the internet. Largely free and uncorrupted by the big corporations.
While SEO was competitive in main industries like law, insurance, travel, etc, it was still very easy for an enthusiast to throw up a informative website for fun in a niche of the internet and gain traction organically. They didn't need to dedicate funds towards SEO.
But over time, the "corporations" have now encroached nearly every corner of the internet. If you want to rank on a search engine for a website you made this weekend to share a fun idea to other people that might be interested, you really can't do it without leveraging one of the many platforms (ie Hackernews, Reddit, Medium, Pinterest, Facebook, eBay, Thingiverse, etc) which subsequently own your content and use it to further spread the gap between the corporations and "everyone else". This is like rich property developers that roll into a community to tear down old houses and then subsequently rent back apartments to the very people they robbed of their land.
I think there is a serious gentrification that is going on here. The only people that might not see it are those of you that are too young to remember the original internet. The original internet was magical. You could have an interest in HAM radios and build a website for fun that could gain SEO traction organically and be discovered simply because it was good and valuable content. That rarely happens anymore. There are very few, if any, corners of the internet that are that "free" anymore. Now, if you are seriously interested in sharing knowledge you either need to turn to the platforms or you need to turn your simple hobby into a sprawling business so you can dedicate funds to competing against the big guys.
Note: I use the term "corporations" here very broadly. But the truth is that 95%+ of the people reading this blog make their sole income thanks to these "evil/scary/bad corporations", so take it in stride. I am just demonstrating the idea of gentrification, which requires separating the "haves" from the "have-nots". But it isn't necessarily good or bad to be on either side of that battle. I think everyone reading this has feet on both sides of the war.