Most source code packages, including their metadata, are generally small even as an aggregated compressed file, and it's very common to download a lot of them all at one time once you've resolved the necessary dependencies. It depends on several factors though -- including community ones (are lots of tiny packages encouraged?) and how things like the build system work when downloading things.

In practice, it's very much an appropriate use case for multiplexing, if you ask me. But not having it isn't a dealbreaker either, IMO. It's a bit more work to support HTTP/2 and can be done rather transparently later on anyway, since the underlying transport can be switched and has an upgrade path.