The problem is the algorithm requires data transfer between threads/machines. The first one can be done relatively easily, while the second one (global all-to-all communication) is not even supported by Swarm at all…
P.S.: Swarm is the root cause of many “bad” designs of current Lightmass. It just grabs jobs from the editor, estimates their costs and distributes them to machines, and finally gets them back. No data transfer is supported in the middle of a lighting build, no global synchronization - it lacks many common parallel computing primitives. If we had them then we can solve many problems such as highly uneven distribution of large ( >1k ) lightmaps, seams and so on. However I’m not strong in parallel computing, I can only hope someone else can do this.