The problem is that direct updates to testing are currently not being tested at all. They are introduced into testing-proposed-updates and nobody has this enabled.
Hence we require updates to flow through unstable. We are not reviewing all what's uploaded to unstable, although many of the uploads there are now voluntarily reviewed to see if they fit the release criteria for migration to testing at this point. Any upload to unstable still ages there, too, just like in non-freeze times. This means that the worst bugs that are introduced by bug fixes are likely to be found by unstable's user base before they hit testing. In turn this makes testing more stable and solid to be used at this point before it's finally released. It also allows some more intrusive RC bug fixes to be accepted under the premise that we'll soon find out if they're flawed.