The other vendors choose when/if they merge Google's commits. The grandparent's comment made WebKit sound like a single entity when it's actually several competing companies with overlapping but also differing views on things. They just happen to be working from a common code base.
Some of Google's engineers have Webkit priveleges. Webkit is a single rendering engine - this should not be confused. It is used by several browsers, who of course, use slightly different compilation configurations, but I doubt that the web designer ever needs to concern themselves with that!