Author here. No threading currently. Once we have all the bytecodes executing, we will perforce be obliged to add threading. Starting with the heavier threading that Java pre-Loom uses.
However, inside Java it may look like the original Threading API. In that case you'd have a JVM running the Thread abstraction in Java on top of green threads in Go, while not supporting Java green threads, and there's no contradiction or even anything weird going on. Perfectly normal.