if you actually read the Ecma response, you'll see that TC45's position is actually quite the opposite. Harmonization is not as simple as just adding a few tags here and there. It's going to be a lot of hard work, and the German Standard Body (DIN) is already working on the first step, which is to identify the differences. This isn't something to take lightly.
Here is Ecma's full response to this issue (emphasis added):
There are currently several XML-based document formats in use, each designed to address a different set of goals or requirements. These include ISO/IEC IS 26300 (ODF), China's UOF, and ECMA-376 (DIS 29500 – Open XML). All these formats have numerous implementations in multiple tools and multiple platforms (Linux, Windows, Mac OS, hand-held devices).
The Ecma Response Document from the Fast Track 30-Day contradiction phase for DIS29500 addressed the question of harmonization by explaining the differences between the ODF and Open XML formats as follows: