Elsewhere in the threads here is a discussion of Nodelets, which are often used to load & run trusted code on ROS inside of host processes. This is like a jank special homebrew version, with a lot of pain, of what you get from wasm Sandboxes, where you can quickly spin up cheap fast runtime and connect them as nauseum.

That's really the key. Sandboxes are really about having many (typically different) runtimes in process. Often these will be processes working with processes. As I highlighted already extensively, the benefit is performance & overhead, versus native processes & native ipc. Sandboxes are just a known term for a runtime within the process, for a subprocess.