At first, yes, this is necessary. Or at least we need to have those binary stage0 tarballs in the srpm, then we can do without network for the actual build. Once we have that rpm completed though, then we need to do it again with an srpm that does not have any binaries, instead using that new distro rustc with --enable-local-rust and/or --enable-local-rebuild. For every new build from there, we'll need to not have any binaries in the srpm, else we have to do this bootstrap again.
It's a bit messy, yes, but I'm hoping to keep it from getting any messier. This scheme is in accordance with Fedora guidelines, and a big reason for #29555, #29556, and #29557 from The Plan.
For Cargo it will be similar -- bootstrap once with a binary snapshot, then build again using a source-only srpm and --local-cargo. But we do have to decide how to package all of its crates.io dependencies too, if we want a successful offline build.
FWIW, I know I keep speaking sort of hypothetically, and Fedora packages have been a long time coming. But I am finally working on it for 1.10 (spec, copr). Even if it still takes a while for that to get into Fedora proper, I intend to actively track that goal there.