Probably just has a conservative Boehm-ish GC. Their website is a bit sparse on the GC details from what I can tell, but anything else would be rather surprising.
"The basic algorithm is Deferred Reference Counting with cycle detection.
# This is achieved by combining a Deutsch-Bobrow garbage collector
# together with Christoper's partial mark-sweep garbage collector.
#
# Special care has been taken to avoid recursion as far as possible to avoid
# stack overflows when traversing deep datastructures. It is well-suited
# for soft real time applications (like games)."
