"(Edit to add: a friend in the contracting industry points out that it also prevents vendors who won't ship GPLv3 from using external contractors to work on Mir - they have to go to Canonical, because only Canonical can relicense contributions under a proprietary license.)"

Works for hire aren't considered distribution (in my experience), so a vendor could hire a contractor to write code for Mir. But if the vendor did not want to distribute that code under the GPLv3, then would have to get a different license for Mir from Canonical before they could incorporate the contractor-written code and distribute the result. This doesn't change much though, if they didn't want to distribute under the GPLv3 they would have to get a separate license from Canonical anyway, whether they modified Mir or not.