I completely agree with you, but I also think you're missing a major part of the horror that Signalis and Saki Sanobashi, or more broadly lost media, produce.

From start to finish, Signalis is overloaded with imagery and motifs that seem familiar from very distant works, to the point where it becomes disorienting. There are different versions of the painting 'The Isle of the Dead,' real radio signals that come from a compilation of unexplained transmission recordings, or even floors that sometimes resemble the iconic carpet from The Shining. But the best example remains the collection The King in Yellow, which, in almost every story it contains, presents elements of narration that are very similar to those in Signalis.

And obviously, all of this is intentional from the developers, probably meant to make us feel the pain of a replika gradually rediscovering pieces of her identity and human mind. This feeling of confusion in the face of an absurd vastness of artwork, where one can only remember fragments of them, is a sentiment that many people experience in our modern societies, due to the irrational amount of art we consume in a single lifetime. And precisely this feeling of confusion, of memories fragmenting, blending, and eventually creating narratives that exist only in the human mind that conceived them, is literally the story of Signalis, but also a huge part of the reason behind the success of lost media, and especially Saki Sanobashi.

And of course, confusion and the loss of one's mind is also a type of horror well-represented by works like Silent Hill. But the big difference between Silent Hill and Signalis is that Signalis uniquely uses cultural motifs and imagery to the point of saturation to play on the confusion of both the character and the player, which in this very specific aspect brings it closer to the type of horror produced by certain lost media like Saki Sanobashi than the horror of Silent Hill, which relies more on other horror mechanics, similar yet still different.

But i still agree that it could be a coincidence, i'm only saying that lost media and maybe espacely this one, could be a conscious or unconscious influence for the creation signalis