Indeed, that's the first time I saw it. Quite unexpected given their past stance.
But I think some legal shifts helped there. Patent trolling cases which tried to attack VPx were repealed (like one from Nokia), so may be MS feels less threatened now (and new CEO helps as well I guess).
According to the spec, Thor is a more traditional block-based codec with quadtree subdivisions in the vein of H.264 and forward.
Beyond that the design is rather conservative and different from Daala's "start over from scratch" methodology. It doesn't look like the two would be poised to merge directly a-la Opus at this point, but rather to be mutual test beds and borrow and exchange unencumbered ideas that work well.