It just means video will still be in H.264 and that browsers without native support will use plugins, most likely Flash. WebM is sure to die unless Google starts sinking a ton more money into pushing it on people. But it's an apparent patent powder keg according to experts in the field. If Google was absolutely confident of its defensibility it would provide indemnification to those implementing it.

If you base your business on WebM you're opening yourself up to being sued out of existence down the road.

The only openness Google likes is what it owns and can control. Microsoft's evilness at its peak was nothing compared to what Google is headed for. Mark my words.