Of course you can get those with `apt-get source` or just grab them from http://archive.ubuntu.com/ubuntu/pool/ directly, e.g. look for .orig.tar.xz, .dsc and .debian.tar.xz files in http://archive.ubuntu.com/ubuntu/pool/main/c/coreutils/. Not really sure who downloads all source in ISO form, seems rather pointless.

Ubuntu (like Debian) has always had online source repos available; they are in no way restricting access to sources here, just deprecating a legacy channel to deliver them. See `apt-get source` subcommand

They do. Source ISO images are a relic of the fact one of the ways sources would be distributed was on a physical CD.