RenameCollection stops migrations here and they are reenabled here as part of the commit to the configsvr. At this point, the balancer can start issuing migrations, although they'll get blocked behind the shards critical section. As soon as the critical section is released, migrations can start.

The problem is that the UnblockCRUD phase may need to be repeated in case of participant stepdown. At that point, a migration could have started, and the rename operation would hit this tassert when attempting to release the critical section (that had already released, and the migration has taken now)