You’d need to use @floop ThreadedEx() for and then use aa += bb (no @reduce). For this code, you can also just use Threads.@threads for.
This is not the intended pattern for @reduce but it’d be nicer to give less cryptic results. I need to debug this.
FYI: @floop for without @reduce defaults to single-thread because of some historical API. I probably should stop doing this.