Don't apologize, you're correct, he's wrong.

The link he posted does not say anything about mussels having a central nervous system (i.e. brain). It says they have a central ganglia.

A central ganglia is not equivalent to a CNS. Ganglia are specifically peripheral nervous structures.. They are nowhere near complicated enough to do any of the processing a brain does.

The "central" ganglia refers to its location is in center, it does not mean "centralization" of nervous function akin to a brain.

The central ganglia is no more a brain than any of the other ganglia in mussels. Your hand has lots of nerve ganglia, there's no reason to assume your hand feels pain when you cut it off because pain is generated in the brain, not the peripheral nervous system.