Point 5 is sound, and it's actually impossible for a configuration of neural fibers to refer to something outside itself (and therefore, if physicalism is true and thought = neural fibers, it's impossible for a thought to refer to something outside itself, which would falsify point 2). Here's why:

The reference can't exist in the thought if "thought" and "a particular stimulation of neural fibers" refer to the same thing. There is no reference in the fibers. You can't "encode" a reference to something else in the physical brain (or any part of the body).

This is because a reference must in some way refer to its object (obviously). But a reference can only be referred to its object by something else. The word "tiger", or a picture of a tiger, refer to an actual tiger only when there is a mind to give them that meaning. But "a particular stimulation of neural fibers" cannot refer to any object, because there is nothing that can give it that meaning. A word or a picture or anything extra-mental can be given meaning by a mind, but when we are talking about the mind itself, this is impossible.

If meaning is given, it's a structural property of mind and is encoded in brain like any other structural property.