This definitely happens with iMessage, as I've seen it firsthand between my kids iPhone and an iPad I use that access the same iTunes account. As to whether KAC was aware or unaware of the potential eavesdropping I'm not yet clear. On the one hand it would make her marriage make more sense, but on the other it could be just on sting to "root" out a leaker.
I'm not clear why Kessler would be writing any statements KAC. And further, in an environment like the WH, who would hand over their laptop with all their accounts accessible and then step away while they blasted their boss to several recognizable media outlets / personalities all within a short period of time.
If any of this account is true, it sounds more like a sting.
https://www.theroot.com/is-kellyanne-conway-the-white-house-snitch-1832017591