Not directly, but it
Okay, so let's start with kyoryu's rule. People make reasonable decisions based on their goals and knowledge.
Why would someone tell the Seelie what the Fomori are planning?
Well, they would if they wanted the Seelie to help, they had the information, they knew that the info would get the Seelie to help, and they knew that giving the info wouldn't harm them.
We can reasonably safely discount the first one - there were there to get help.
They had the information, but if they didn't remember the info, that would explain it.
If they didn't know the information would get the Seelie to help, that would also explain it.
If they had reason to suspect that giving the info might bite them in the butt, that would also explain it. You know it wouldn't, but they don't. And I'd say based on your descriptions of things that your games have a high tendency to have unforeseen blowback from seemingly innocent actions - at least that's the impression that I get, and I suspect your players would agree. That kind of gaming, even if it's not the case here tends to breed players that are very careful about what they do, since they don't know what is or is not a land mine to avoid stepping on.
So that gives us a few explanations. First, and simplest, is that they just forgot about the attack. If they forgot about it, they wouldn't mention it, even if you told them.
If they didn't know it would help, and especially if they also didn't know it woudn't harm them, they might also be cagey with the info. To them, the attack was effectively irrelevant - they didn't know where the Seelie had a stronghold, and couldn't know. If the Fomori were instead planning on attacking something completely unrelated to the Seelie, would you have been as confused as to why they didn't divulge that info? That's understandable, but if so, that's very much a clue that you're expecting them to make decisions based on your knowledge.
IOW, we can rephrase the quesiton from a few potential POVs of theirs:
1. Why wouldn't they give the Seelie information that they didn't remember? This is obvious.
2. Why wouldn't they give the Seelie information that they didn't think would be likely to be beneficial to them, but which might actually have a higher chance of being detrimental if they gave it out? There's a numbers game here, but it seems reasonable.
The point is that when you have to create a scenario with three possible solutions, you tie yourself less to any of them, and often can become more open to the fourth solution.
That may not work for you, but I find it works for a lot of people.
You should ask yourself why they act this way. It's likely something to do with past experiences and learning from them.
That's not what I said. This is a very good example of not appearing to try to understand the point.
The point is to search for intent, with the understanding that table communication is low-bandwidth and lossy compared to what's "actually happening".
You're taking the instructions fairly literally. I'm suggesting that rather than doing so, you understand the intent of the instructions, and interpret said intent charitably. If the intent is unclear, clarify it. This is because there is a communication gap between all people, the character knows how to interface with the illusion more effectively, and the character has more time to do so.
This feels like a strawman.
Doesn't matter. They're authoritative over their charcters, not you.
I think that's fair, but in the "sounding the alarm" case I think it went against the original intent.
There was also a quote somewhere about "that's not the GM's job". And I'd really like to address that.
I'd highly suggest you get more flexible on what you think the GM's job is. What's important, having a fun, satisfying game with your friends? Or your definition of the GM's job? If what you're doing isn't working, do something different.
Also, nobody is saying "prevent the players from making mistakes". We're saying "make sure the players understand the situation enough and are clear enough on what the characters would know that they don't do things the characters know would be dumb." That's an entirely different thing.