I do not have Millman and Halkias and cannot access it from the link you gave. However, the excerpt you quote suggests that we are talking about two different things.
In the one case, your original post stated that diffusion does not occur in metals. With reference to diffusion of metal atoms from one metal into another, that simply is not true. Such diffusion does occur, albeit slowly compared to liquids and gases, as pointed out by myself and others in this thread.
On the other hand, it appears you are really talking about the mechanism of electrical conduction in metals versus semiconductors. I am not knowledgeable in that area, but it seems entirely reasonable that there are differences in the predominant mechanism. See:
Fermi level - Wikipedia, the free encyclopedia
Source:
Semiconductor - Wikipedia, the free encyclopedia
The effective mass is important as it affects many of the electrical properties of the semiconductor, such as the electron or hole mobility, which in turn influences the
diffusivity of the charge carriers and the electrical conductivity of the semiconductor.
Finally, I would interpret the simple phrase, "Not ordinarily encountered in metals," to mean that the mechanism for conduction being referenced may occur in metals or cannot be excluded as occurring in metals, but it is not the predominant mechanism in metals. Unfortunately, without access to that book, I cannot say more about the context of that statement.
John