Yes, dependent packages should use %{fpc_arches}. FPC itself doesn't do that.
I imagine the reason is that this allows us to add new architectures to FPC and do some
trial-and-error builds of FPC without affecting dependent packages - if FPC itself used
%{fpc_arches}, then adding new architectures to FPC would require updating fpc-rpm-macros,
and that would make all dependent packages fail to build, since the new-arch builds would
fail with "Package not found: fpc" until we got FPC available on those.