On further reading I see that it namespaces everything more completely than I thought. You don't get to make things default with alternatives, it seems to use a PATH wrapper 'scl' command.
It relies on Red Hat's COPR rather than providing an alternative to it, so it doesn't add anything new or interesting for maintaining normal package trees. COPR doesn't help with building srpms, versioning and maintaining trees, etc, it just does builds and repo maintenance. Useful but not exciting; it's a pity it can't build SRPMs from specs too.