I think the article is wrong. The RAE accepts indistinctly after-sales and after-sales. You just have to enter after sales and you can see the answer: also after sales. When the RAE does not admit a word due to its non-existence, the following legend appears:
The word xxx is not recorded in the dictionary. The entries shown below could be related: xxx, xxx.
The correct definition of after-sales or after-sales, that is, what appears today in the RAE is:
F. Period after the sale of a product, in which the seller or the manufacturer guarantees certain services, especially repair. After-sales department. U. m. in apos. After-sales service.
