Yes, you end up with a tiny DSL for specifying transformations to make to a child process: they key difference is that the kernel can execute this DSL much more

Yes, you end up with a tiny DSL for specifying transformations to make to a child process: they key difference is that the kernel can execute this DSL much more efficiently than it can code written in the host language: fork closes over the entire world, and posix_spawn doesn't have to do that.