[Unison-hackers] How to enable multi-threading when both roots are local

[netawater]

I have study the source code of unison-2.31.4:

transport.ml:23:let maxthreads =
transport.ml:24:  Prefs.createInt "maxthreads" 20
transport.ml:33:let actionReg = Lwt_util.make_region (Prefs.read maxthreads)
transport.ml:74:  Lwt_util.resize_region actionReg (Prefs.read maxthreads);
transport.ml:75:  Lwt_util.resize_region Files.copyReg (Prefs.read maxthreads);

It seems maxthreads always being enable, but unison run in
mult-threads even I set maxthreads as 1

Thank you very much!