The point is not that the characters would have known that, it is that they needed to know it to realise the significance of the plan to attack the woods. It is a common blindspot for GMs that things that are obvious to them are decidedly non obvious to their players even if the players do have all the available information, and doubly so when they do not.
If Talakael's players are anything like he describes them here, at least three of them are "obnoxious jackass[es]", and the best thing Talakael could do to improve his gaming is kick them into touch and recruit better players. He is either unwilling or unable to do that, but whether he does or not, that does not mean he cannot look to get his own house in order and improve things on his side of the screen. He can and should IMNSHO!
Huh? Has there been any suggestion from Talakael that the Fae in question did not believe that Black Spiral Dancers and Fomori were a real thing in the setting? If so, I missed it.
The reason a child talking about supernatural stuff is not believed is because supernatural stuff does not exist. But in the World of Darkness, stuff that would be supernatural in real-world terms absolutely does exist. The Fae definitely know that they exist!
It depends what "this way" is. I strongly feel that consent is important and can be withdrawn at any time. I strongly feel that talking in character is its own reward for those that enjoy it, and should not be forced on those who do not. Despite multiple posts on the subject, I am still not confident how you stand on those issues.
Conversely, I am quite* confident that Keltest was not advocating removing all dialogue for everybody because one player was uncomfortable. Just that such a player should neither be badgered into talking in character, nor penalised with increased difficulty for not doing so.
(* That's a British "quite" - roughly translated it means "almost utterly").