In general, this question is "too broad..." because there's a lot of games and communities of gamers outside the D&D domain and you're asking about the problem of normative textual sourcing in general. But here's the deal somewhat specifically to this situation with some more general philosophy mixed in.
It comes down to the community of play around that game (as I point out in my answer to that question). For a variety of reasons a substantial chunk of the D&D 3.5e rules folks out there have decided "no we don't accept the FAQ." So they don't.
That doesn't mean you can't accept the FAQ, or that others don't. Feel free and use it in your game and here in questions and answers.
However, on a site like this one with votes driven by the community, those super against the FAQ can and do downvote answers that justify themselves based on the FAQ because they don't personally find it useful (even though the OP might). I don't agree with that, but one of the tenets of SE is "people can vote however they want for any reason they want - we can provide guidance on how to use your votes usefully but in the end users click the button they want to click."
In the end this is a religious problem. As gaming is distributed, there's no "authoritative source" per se - the company may have a stance, groups of gamers may have a stance, none of it really matters. The only circumstance in which it matters is in Organized Play events where the controlling group says what rules are normative in that situation. You can "not like" the FAQ but if Living Greyhawk uses it, then you are using the FAQ for that context. There is no meaningful answer to what textual authority is normative outside the rules of the specific gaming group for any game.
This isn't a discussion forum so I'll give the bottom line answer from the RPG Stack Exchange official policy perspective - there is no definition of "what's official." A source is never unacceptable, it just may pull downvotes from certain users. The D&D 3.5e FAQ is not "officially off limits" and you are welcome to use it, with the understanding that there's a group of people who don't accept it as normative. The 3.5e FAQ specifically seems to be one of the "hot-button" topics that cause conflict and mutual downvoting but that's not the case for pretty much any other game, their FAQs, their rules, etc., so the more general case isn't as much of a bother.
I do wish voters would be more... discerning in their downvotes. Not liking some of the rulings in the FAQ, or not considering it "letter of the law" normative because the books say "only other books can supersede me" is fine, but discarding the whole thing as "certainly there is no helpful guidance of any sort or peeks into RAI it could provide, I will just downvote any answer wherever I see the word FAQ" is not the level of intellectual engagement we'd like to see here on the site.
But I think that's a problem that will fix itself over time, as 3.5e recedes and newer D&D versions are deliberately taking a less legalistic approach to the game, so that arguments of this sort are harder to have.