Is there a reason this specfile goes against the packaging guidelines and doesn't use the proper macros? I was going to submit a PR to fix the issue, but I figured I'd check first to make sure there isn't some legacy reason that this specfile doesn't use the macros.
It is a rather old spec file (the first revision was created in 2001). In the long long ago, there was a difference between /sbin and /usr/sbin. Since %{_sbindir} is /usr/sbin, it was hardcoded in the spec file like that, because you wanted busybox bits available even when /usr was not mounted (yet). Obviously, this is no longer the case (/sbin is now a link to /usr/sbin), so I've gone ahead and fixed the macros in the 1.35.0 build in rawhide.