i think they're trying to say that:
isolation doesn't = punctuated equilibrium but it is one of the reasons for evolution, be it gradual or 'punctuated equilibrium'.
isolation is often compounded with a different environment for each of the seperated groups of organisms - the changed conditions in isolation cause natural selection to their specific environment. this happens in either of the proposed theories of evolution.
ok - well that's what i think (in very long winded terms). feel free to prove me wrong