The confuses a necessary condition (having central nervous system) for an organism’s possessing a capacity (capable of planned locomotion) with a sufficient one. This overlooks the possibility that an animal can have a central nervous system without planned locomotion.

There are no assumptions about sending messages “from their central nervous system.” Although the author does assume that organisms with a central nervous system are capable of planned locomotion, that doesn’t imply the author thinks the nervous system sends message to muscles.

The argument doesn’t make assumptions about what is “biologically useful.” The argument is based on a misinterpretation of the fact that a central nervous system is necessary for planned locomotion. What is biologically useful has no part on the reasoning.

The argument doesn’t make assumptions about what is “biologically useful” or the original purpose of a biologically useful adaptation. The argument is based on a misinterpretation of the fact that a central nervous system is necessary for planned locomotion.

The argument doesn’t make any assumptions about a “rudimentary nervous system.” We don’t know whether any organism has a rudimentary nervous system. It’s not clear whether a central nervous system is rudimentary or not.