What the assessor has on record and what actually exists sometimes don't line up. : )
I suggest you contact the real estate agent, home inspector, or seller of the property . . . or look yourself, if possible . . . to see if there is any indication of a central ac set up (working or otherwise).
If it is a matter of home value or price, talk to your agent or loan officer to see if everything is in line for the wall unit, no matter what the assessor and appraiser are calling it. If the seller promised central AC, you might have reason to negotiate the cost of getting one.
If it is a matter of tax assessment, I suppose they have the right to call it anything they want - but I'd visit the local assessor's office for a good, thorough explanation of what is and isn't considered central ac for tax purposes.
I will say, though . . . you are buying the house as it is - so whatever YOU call that wall unit is the most important. Just know what you are getting. The change in value (appraisal for lender or assessor for taxes) isn't going to be too drastic in most areas as to affect your transaction - just know what you are getting and be ok with it before you close.