As to Google I'm pretty sure it only supports a subset of OOXML since even ODF isn't supported by any toolkit 100%, last I read LibreOffice was adding minor missing ODF functionality, and as I read a while ago even Microsoft wasn't fully supporting OOXML since it's too much work.
In Google's case one has to notice that there's many levels of support for something:
1) partial support, even that partial part might be crappy
2) full support, but crappy or not optimized
3) full support, good and optimized
Does Google state which type of support for OOXML they have?
And yes, "native" basically doesn't mean anything. Remember how Microsoft praised IE9 for "native" hw acceleration?