It's not an Orcish Maneuver.

The original example +* (reads "numeric context Whatever") is syntactic sugar for the one arg closure {+*} which is in turn treated by the P6 sort builtin as a key extractor and results in use of the comparator closure {$a<=>$b}.

I've realized I didn't make it clear that my response to your challenge contained two parts and you needed to read both to see the ST equivalence. I suspect you didn't read both. I've updated it to try make things clearer.