How is that a moat?

All that matters is that the CDN is good enough to serve viewer traffic. If your CDN isn't up to snuff, the outcome is that the viewer can't play the video they want. AFAIK, no Netflix competitor is suffering from insufficient CDN resources.

Sure, some choices make the cost of CDN higher or lower, but it's not material enough of an impact on competitors to matter.

The actual moat for streamers has been, and always will be, the breadth and popularity of their exclusive content. Netflix will die if they don't continue to produce exclusive content that audiences will subscribe for.

Everything about a cdn is cost per MB, and Netflix has that priced at I don't know, I imagine almost zero, and maybe negative because they host with ISPs, which other cdns can't do. Which means their competitors must spend infinitely more money until they hit a scale where they can start to follow in Netflix's parh. Also, I imagine that buying hardware now, especially DRAM is four or more times as expensive as it was when Netflix paid for a lot of their hardware, so yeah, that.