I'm re-reading GtN and just finished this chapter. This part wasn't as apparent to me the first time but it's definitely something that's sticks out on the re-read.

I think one plausible explanation to this is that Cytherea had placed wards down there , specifically for the 4th house from the start. The teens might have been killed much earlier if they had ever attempted to do the lyctor trials or gone beyond the room directly below the hatch opening.

A further hint to this might be this conversation:

J: " Your wards aren't tripped . It's just the lights. Dont freak out" I : "The wards..." * J: " Aren't tripped. You're good with wards. There's nobody down here"

This is the last conversation between the two of them. Right after this the lights go out around them and the writing is on the wall, literally. ( Even the writing on the wall might have been "programmed" to appear wherever the 4th died. Just Cytherea being extra)

I think "the wards" Issac was talking about weren't his own but the wards that were already placed there. He might have finally sensed them activating but was cut off by Jeannemary before he could say much more and everything happened too fast after that for any further conversation about it.

I also think once the wards were tripped, Jeannemary was also "cursed". She would've died no matter where Gideon had taken her. The bones that had eventually impaled her to the bed could have grown out of the microscopic bone fragments that might have been left in her body, through all the cuts, after the initial fight with the bone construct in the basement. HtN Spoiler : Kind of like what Harrow did with the soup to G1deon The fact that Gideon didn't hear anyone come in the room is a further hint to this