A simplistic definition of physicalism is that physical matter is all that exists. However, it is pretty clear that other things do exist, and so I am sure that IMS does not hold to physicalism in this sense. If JBSptfn is using physicalism in this sense, he is guilty of a straw man, so let us assume he is not.
From Jim's link:
"
Physicalism is the thesis that everything is physical, or as contemporary philosophers sometimes put it, that everything supervenes on the physical."
And later:
"
If physicalism is construed along the lines suggested in (1), then we have an answer to the completeness question. The completeness question asks: what does it mean to say that everything is physical. According to (1), what this means is that if physicalism is true, there is no possible world which is identical to the actual world in every physical respect but which is not identical to it in a biological or social or psychological respect. It will be useful to have a name for physicalism so defined, so let us call it supervenience physicalism."
All our senses are physical (in this sense), our eyes detect photon emissions, our eyes detect vibrations in the air, etc. Whatever we can sense must, therefore, be physical.
If ghosts exist, then to be detectable by the senses they must be capable of interacting with the physical world. Okay, but what if they only do so intermittently? Perhaps only when they choose to? Such a ghost would be detectable by the senses, but could not be successfully studied by science. Popper defined a physical proposition to be one which can at least in theory be denied by observation (see
here). I can claim there is a desk in the room, and this can be denied by the obserrvation that the room is free of desks. I can claim there is a ghost in the room; this cannot be denied by observation as we cannot know if the ghost is present but choosing not to manifest.
That does not mean physicalism is necessarily true, only that science is restricted to it. This is generally called methodological naturalism.
https://plato.stanford.edu/entries/naturalism/#MetNat