Neither Reddit or Twitter are in any form shape or fashion a “monopoly”.
Twitter and Reddit are just looking to get a share of the AI profits. Is it a monopolistic move? Maybe, I don't see where else the AI companies will be a able to get good structure data. The move is pointed towards the AI companies, specially new companies, not users. Since limiting user input will destroy the business model that sells data for AI companies. Going forward structured quality data will always have a price.
I expect for AI companies to buy twitter and/or reddit in the future so they aren't kept away from the data they need.
The alternate move is for AI companies to create their own products that they can use as a data pipe similar to twitter and reddit.
So what happens to them or their APIs is not important and they are not powerful in any real sense.
In comparison, Standard Oil was a real, powerful monopoly that controlled a key resource.
In the "tech world", I'd say Google is the one that matters because it has become the gatekeeper of the Web and Google ranking is critically important.