No, the claim is that an explanatory framework limited to physical terms is not complete enough to encompass mental events.
In which case it sounds like there should be some mental events or experiences that are impervious to physical interference. So for example there is no conceivable physical intervention that could affect them. Do you believe there may be mental phenomena that are not causally related to physical processes in that way? It sounds like this should be testable.
I think he is only saying that an empirical language alone will not be enough to relate mental processes to physical ones. You have to define what a mental event really is before you can properly test anything about it, and depending on what definition you choose it might be inherently un-testable. If you define a mental process as being composed of physical processes from the start then of course you will conclude that all mental processes are influenced by physical ones. There are other possible definitions though such as physical processes being composed of multiple mental processes (the opposite of the last) in which case no physical test could be performed to verify a "change" occurred in the mental subject (but a "mental test" of sorts might still be able to tell us)
It's like saying you can't model a football game in terms of the behavior of proteins. Sure there are proteins, and there is no magic anywhere, but the conceptual gap is just too vast.