I don't think this is intentional, but I do think it arises most often from two things:
Idea: We should have this...
Answer: This exists already.
In a lot of cases even just knowing there is an otherwise lesser-known mechanic or item or faction can be an advantage, sometimes a big one. I've learned about a lot of things I've never actually seen IC just by reading through some threads where people chime in to say such-and-such is already implemented. The other scenario that is pretty common is...
Idea: This should be balanced because...
Answer: Yes it should be balanced but I can't say why.
Answer: It doesn't need to be balanced because X, Y, Z reasons.
I think one of the biggest problems on the idea board is players acting as gatekeepers for what are acceptable ideas. If someone suggests an idea that doesn't make sense because of mechanics they don't understand, there's no need for players to educate them on this topic or get into debates over why the mechanics work with everyone talking about how they can't really discuss it but still giving away details simply by virtue of that discussion.
The staff are not just going to add an idea because it sat unopposed on ideas, it's not necessary for players to chime in on things in a way that divulges details, and a comment saying "this wouldn't work but I can't say why" doesn't add anything to the discussion anyway.
Players don't need to know if their ideas are viable or not, they don't need to know if they make sense, they don't need to know if they already exist -- all of those things can be filtered for by the staff when deciding to implement.
All these discussions do is create an avenue for bypassing IC mechanisms to learn about the game. We shouldn't have any comments of players saying 'oh thanks, X is a good idea I didn't know about that' on the BGBB.