The book states “Upon initializing a new module, Go creates a file go.mod in the root of your
module directory” but, at least in Go 1.20, that is not true. It is created in the current working directory.

$ go help mod init
usage: go mod init [module-path]

Init initializes and writes a new go.mod file in the current directory, in
effect creating a new module rooted at the current directory. The go.mod file
must not already exist.

This means that the example of creating a nested directory, initializing it and then showing the contexts of the go.mod work, but the wrong directory has been initialized with the new go.mod file.