Syphon72: Thank you for letting me know. I check GOGDB and the last update was on 11.16.2023. Which I thought was before the holiday content was released.
The builds tab on GOGDB shows the last change to the actual files on 2023-11-16 (Build ID 57048356750520729). That's when the Holiday Content Update went live on a beta branch.
It was then made available on the default channel without changes to the underlying files on 2023-12-05 (when the update was officially announced). That's why there's the "Build added: 57048356750520729" in GOGDB changelog tab on 2023-12-05 (same Build ID).
The offline installer was uploaded a day later on 2023-12-06
Anyway, I can confirm all the games added in the Holiday Content Update have been available and work as intended since back when it was announced.