Not exactly. That's the programming rule for structured concurrency which is a very simple bit of logic added on to the ExecutorService class. Loom itself is just about making threads super cheap/fast. You can use new cheap threads in exactly the same way as normal and nothing will complain. You can also use structured concurrency with old threads, and again, nothing will complain. And you don't need Java support for structured concurrency. You could implement it today with some wrapper classes.
The discussion further down about "scope variables" foreshadows making structured concurrency more strongly, directly supported by the language's runtime:
> What about inheritance? Because SVs are immutable, and because structured concurrency also gives us a syntax-confined thread lifetime, SV inheritance fits structured concurrency like a glove