If you read Richard Haass's book (https://www.cfr.org/book/world-disarray) or listen to the GCSC or Joe Nye you will hear a lot about the value of stability, both in the world and in Cyberspace. But like the real world, when you enforce stability it is like pushing on a tape bubble. What is stable for one stakeholder is oppression or uncertainty for another.
The old sayings about the Navy protecting the seas for large multinationals to exploit the tiny economies of third world countries are also portable directly to cyberspace, and you can see it in the GCSC's first "Norm".
Why DNS? To a technologist, DNS is one of a suite of aged Internet protocols along with SMTP and HTTP. And of course it gets manipulated in many ways all the time - most importantly the FBI will blackhole a name that is being used as part of a botnet, for example. Also commonly, courts will assign names to various companies based on their trademark.
But anyone on the upstream path that is trusted by your browser or operating system can manipulate, and often does, DNS. You may recall the concerns from various ISPs and network providers about DNS over TLS, which would prevent them from monitoring and manipulating their customers' DNS when using certain browsers. Many VPN and security providers filter DNS for you, providing many security benefits.
The UK's now scuttled war-on-porn was slated to use these exact methods to try to filter out adult content. It would, in other words, have violated this proposed norm.
A better and simpler "Norm" would simply be "no DDoS attacks anymore, please". But the ACTUAL NORM is that lots of governments use DDoS attacks whenever they want to punish a company that is posting content outside their legal reach, China, in particular with their "Great Cannon", but also Iran, the US, etc. DDoS attacks often flood core routers, breaking things in bad ways, and so it's possible this idea of "please don't mess with the core" is an attempt to shoehorn in a bunch of unstated things to stop what every country already does.
Not to mention, many countries spend a lot of time hacking routers, which are the very definition of the core?
In other words, from the very beginning the GCSC proposal has severe challenges. No doubt some handwaving will occur in the name of "a perception, true or not, of forward progress".
What really annoys me is that providing guerrilla uncontrolled internet to people is the best way to affect change in this day and age. Imagine the Hong Kongers having secure internet, unmonitored by the PRC? But this conflicts with the essential values of controlling pirated Disney movies from ever being reachable by the market...
Also, to have workable norms in this space, you have to agree on a shared reality. We don't currently have that in our system.