You're taking that passage a bit out of context.
There's a house that the PC is sneaking into. You, as the GM, have an idea of who or what should be in that house, even if you didn't establish a list of inhabitants beforehand. So if the PC fails and startles someone, it's someone who you would consider a logical resident of that house. It's not that a cook materializes out of nowhere; it's that you have decided that sure, it's logical that this house has a cook.
It's no different than if the PC goes into a bar and wants to know if there's a barmaid. Did you actually figure out every single employee of that location? Maybe, but probably not. If you hadn't, would you say "there's no barmaid here" or would you say "sure" and grab a random name generator? Does that count as a quantum barmaid to you, simply because you hadn't established ahead of time?
To me, the ability to say "sure, there's a barmaid" or "you startle a cook" is the type of improvisational thinking that GMs are supposed to do when players do things that the GM didn't expect.