I’ve not finished the book yet so… if later in the book you come back to this example let me know…
In section 1.1 the first example is a fair and simple example. As the book continues, you show the reader good examples where the cut you take of the task hierarchy is at a more consistent abstract level (so far). There is still the potential that the reader may take this first example as exemplary (unless you come back to it later and say why in some places it’s probably too low level).
To give the newbies the best introduction, I think the first example might work better if it was at a similar level of abstraction to your first walking skeleton example.
One approach could be to use your first “2+2” calculator example as your example in section 1.1.
Alternatively, the smallest change I would make is to change the Given and the When: `
Given I have chosen to sign up When I sign up with valid details