Dr. Awkward said:
The FAQ and Sage are not errata. They are therefore not primary sources. It is possible that the OP was confusing "primary source" and "official source."
I'm quoting this one not to single it out, but just as representative of several responses.
1. Is the FAQ a rules source at all?
The attempt to distinguish between rule clarifications and rule sources is doomed to failure. Any rule clarification is itself a rule.
If I say, "Fighting in this way" refers to "if you wield a second weapon in your off hand", rather than "get one extra attack per round", the result is one rule rather than another.
2. Official versus non-official
First, it is not clear that there is any such thing as an "official source". Does the errata or DMG use the term "official"? When people distinguish between the printed books as "official" and the FAQ as not official, they are merely using their version of the primary source rule. That is, their interpretaion of the primary source rule is, e.g., that "clarifications" are not "rules" or that printed text takes precendence over an electronic document.
Second, if anything is "official", the FAQ sure is. It is labelled by WotC as official.
3. What can be a primary source?
According to the errata, "When you find a disagreement between two D&D® rules sources, unless an official errata file says otherwise, the primary source is correct."
The errata does not not limit what can or can't be a primary source. The errata gives an example of what to do if there is a contradiction between PHB, DMG or MM. But this does not exclude the possibility of other sources being primary for other situations.
Again, that's all I've got time for now. Thanks to everyone, including those who take a contrary position, for providing interesting responses.
-Redshirt