They're positioned to have much wider reach than even AWS. You probably wouldn't put AWS in front of GCP or Azure. You might put CloudFlare in front of any of those. And once you've done that, well, they offer these other services that compete with the backend you're already using....

Their position in the stack has a great deal of market relevance, which translates to risk if they realize that potential, because they could well end up being a critical part of more of the Internet than AWS is.

The parent is referring to concerns of a ubiquitous "man in the middle" and you are referring to SPOF. Those are two very different things. At any rate I can always choose two 2 or even 3 cloud providers for diversity against having an SPOF with AWS.