Note that this is a rather narrow view of the scope of things.

Yes, the demonstrator is a python script that opens up 'su' and uses splice+this vulnerability to change it to 'just assume all privileges and become sh'.

However, it's that any process in any namespace can leverage a certain socket type and splice to effectively modify any filesystem content they want. It's easy to see how this could be part of a chained attack to, for example, replace a protected service that is firewalled off with a shell. An RCE in a service permits rewriting nginx in an entirely different container and replaces it with a shell backend of your choosing.

That 'flatpak' application on your single user system that is guarded from touching your files that aren't related? That isolation doesn't mean anything if this issue is in play.

In terms of shared systems, while it should be avoided if possible, practically speaking there's a lot of shared resources.

I don't get why I've seen so many people saying "ehh, no big deal, privilege escalation is just a fact of life".