Mathilde didn't self-isolate after keeping any of her other secrets, and both possession by a hostile god and posession of the Liber Mortis are much bigger deal.
Now, as you say, the daemon may not know about those, and so not realise that keeping a secret like this is just part of Mathilde's normal routine so won't be a problem at all. That sucks for the demon, but is great for Mathilde, who can say nothing and just go about her day while the daemon fumes impotently in the Aethyr with its hopes to mentally unbalance her dashed.
While that's true(-ish) for this possible plan (there's a whole thing about different types of secret and guilt I could go into here), it's not necessarily true for other 'say nothing' plans. Another example might be it wants to feed Mathilde's guilt about not saying anything when another Everchosen does appear in the immediate future, or it could be hoping to show up against while Mathilde has company and say or do something that makes Mathilde react in a way that gives away their prior meeting. Hell, going full speculation, maybe the 'Fated' trait makes us glow with purple fire when it's in play, and all it needs to do is sit back and wait for us to inflict some very inconvenient questions upon outself.
Regardless, my point with the reply I made to the post by DP was that 'doing nothing' could actually be the response that the demon wants. It's not a silver bullet, because we don't know what the thing is actually aiming to get out of this.
Also, as an aside, please don't reply to this post. As politely as possible, you have a tendency to jump on literally everything that could be even slightly inconsistent with your point of view, and throw every argument you can think of at the wall to see what sticks and... well, frankly, it's exhausting to read or be the subject of. This isn't me saying 'stop posting' or anything like that- you have a right to share your opinion and try to persuade others. Just please, for the duration of this vote, stop replying to me personally.
But what if others actually don't trust us. I mean you would not think Thorgrim would answer 'die well' to a request for aid and then he rolled a 2 and in universe he became the person who would do that. Low rolls do happen so maybe don't just plan for the median odds of how people will react, especially when Mathilde actually brought up in character what the worst case scenario might be
If we're talking about a bad roll, then not telling people could be equally as vulnerable. Maybe we get a roll for getting away with lying, or down the road the demon pops up and says something suspicious in front of us and witnesses and we have to roll for poker face, or Kragg gets a roll to notice the wrinkle in reality during his maintenance of the Eye or something. Luck plays a roll, it's true, but it cuts both ways.
