It's not quantum mechanics, it's just an analogy.

In the followup chapter, you e.g. get the following hypothetical:

"Why not just let the 'degree of existence' be a complex number, while you're at it?"

Bo'ma rolls his eyes. "Please stop mocking me. I can't even imagine any possible experimental evidence which would point in the direction of that conclusion. You'd need a case where two events that were real in opposite directions canceled each other out."

Which hypothetical is actually a reference to an element of real QM -- as the amplitudes of configurations are indeed complex numbers, and can indeed cancel each other other. (if I'm not mistaken)