I understood the article to say that there were posters up for all the holiday stamps except the Christian ones.
The relationship between displayed posters and stamps currently available for sale is unclear.
Prior to the release of commemorative stamps advertising posters are sometimes hung announcing the upcoming release. Sometimes those posters remain up for months long after the commemoratives have been sold out.
However, it cannot be assumed that all commemorative stamps are heralded by a corresponding promotional poster.
Promotional posters cost money. If a commemorative stamp is projected to "sell itself" with little to zero promotion, then what Postmaster would want to spend more taxpayers' money promoting such a popular stamp?