This is true, which is not particularly useful when deciding if a feature is available. Using the user agent to make this decision incorrectly assumes that all user agents of a particular browser+version all support exactly the same set of features. This is obviously not true.
The user agent may be meaningful[1], but the meaning that it communicates isn't feature data.
[1] If you take the header at it's word; as the bug being discussed demonstrates, user ages may be lying.
But it kind of is (to some degree, at least). VP9 was recently added to Firefox 28. If the User Agent says the browser is Firefox 27, then you probably shouldn't try serving VP9 to that browser.
Obviously the User Agent doesn't communicate the complete feature set. But some useful inferences regarding features can be derived from it.
You might object to the form of the communication, but that doesn't render the communication inexact. Just think of it as browser slang.