it seems to me that most of those questions should influence positively employee happiness.
Correlation just means that when goes up, the other tends go up (or down, as the case may be). It doesn't make any claim on whether there's a direct influence or not. Hence the common refrain: correlation is not causation.
You could hire a bunch of jolly simpletons, have the happiest company in the world, and go bust before the year is out.
You could make it perfectly clear what is expected of everyone, give them all the kit they need, implement all their valuable suggestions then depress them by paying them minimum wage and discouraging fraternisation and have a similar outcome.
I’ve read the book, and it’s a good one.