Yes, it means that the processes startd by that new make count towards the job limit of the parent process.
I guess the + is not needed here. I'm not really sure if "make -j 4 clean" makes sense and if it helps here.

Last time I aded that was in commit 7c748e2b:
-MAKE_ORIG = $(MAKE) -f Makefile.orig
+MAKE_ORIG = +$(MAKE) -f Makefile.orig -j$(J)
 
I had to do that to avoid the "warning: jobserver unavailable: using -j1." messages.