Most of the APLs don't automatically parallelise array operations either, because it's hard to know automatically when it's worth it. This is exacerbated by the fact that most APLs don't have a compiler, so the granularity of independent operations is fine.
While I don't know much about the mysteries of Ks implementation, I know that the most widely used industrial APL implementation, Dyalog, uses a pretty conventional explicit task parallel API for parallelism. They call it "isolates", and it's essentially about launching a thread that has its own internal APL state (with a lot of polish for convenience and communication, of course). There may be a few primitive operations that are automatically parallel internally, but they are rare.
While I don't know much about the mysteries of Ks implementation, I know that the most widely used industrial APL implementation, Dyalog, uses a pretty conventional explicit task parallel API for parallelism. They call it "isolates", and it's essentially about launching a thread that has its own internal APL state (with a lot of polish for convenience and communication, of course). There may be a few primitive operations that are automatically parallel internally, but they are rare.