I believe the spec says arguments can be deleted in any order, so it always chooses "current directory” first, which fails, failing the whole command

No, doesn’t work like that, meaning it should actually delete everything. The shell is expanding /* to the list of paths, not rm, and /* does not expand to / at all.

Try adding “echo “ in front of the rm command line to see how it expands. (That, by the way, is often a good idea.)