According to the same document, in case developers start focusing on all the web browsers, they will have to wait multiple years to get a direct Harmony support.
Here is an excerpt from the document:
What will Google developers be using?
We will strongly encourage Google developers start off targeting Chrome-only whenever possible as this gives us the best end user experience.
Developers who can focus solely on Chrome can expect to be able to see some Harmony features in Chrome (behind a flag) by the middle of 2011. Developers focusing on all browsers will have to wait multiple years for direct Harmony support, due to the relatively slow pace of the standardization process.
Whether or not Harmony claims are true, remains to be seen, but in case they are, Google’s position is pretty much justified, if you ask us.