I don't know what the take away is supposed to be here. Everything spoken about here is already basic network programming. It seems to focus on higher level plumbing and geeks out on state management even though this is just a matter of preference and has nothing to do with networking.
The most interesting thing I learned from the article is that cloudflare runs a public stun server. But even that isn't helpful because the 'good' and 'useful' version of the STUN protocol is the first version of the protocol which supports 'change requests' -- a feature that allows for NAT enumeration. Later versions of the STUN protocol removed that feature thanks to the 'helpful suggestions' of Cisco engineers who contributed to the spec.
The big thing, in the context of Rust, I think is how this solves function colouring, but it also makes testing really simple as outlined in the post.
The current situation in Rust is that if you implement a library, say one that does WebRTC, that uses the Tokio async runtime. Then it's very cumbersome for folks to use it if they are doing sync IO, using a different runtime(smol, async-std etc), are using iouring directly etc. With this approach you don't force the IO choice on consumers and make the library useful to more people.
The parallels with abstracting over the effect type and Free(r) monads are really apparent if you've had exposure to that style of programming. As you said, the benefit is that you can separate the business logic (what you want to do) from the execution model (how you do it) and this is very much an ongoing theme in programming language development.
The most interesting thing I learned from the article is that cloudflare runs a public stun server. But even that isn't helpful because the 'good' and 'useful' version of the STUN protocol is the first version of the protocol which supports 'change requests' -- a feature that allows for NAT enumeration. Later versions of the STUN protocol removed that feature thanks to the 'helpful suggestions' of Cisco engineers who contributed to the spec.