Project Loom is both about the JVM and the language Java. Most of the work for Loom AFAICT is at the JVM level, and the benefits are that all Java code will Just Work(TM) with the new primitives underneath them at the end.
Scala’s varied async/concurrency libraries are implemented in user land, and still use threads underneath. Mechanically, you must opt in to these and have to work to interop with code that might use other primitives. Scala can handle a lot of this complexity at compile time w/ types, but it’s not perfect, and certain runtime behaviors will always be out of scope.
Loom improves this by allowing any language that runs on the JVM (Java, Scala, Clojure) to opaquely use virtual threads to run their existing synchronous, scheduler-unaware code on the new Loom concurrency primitives implemented in the JVM. That’s powerful!
