I believe it applies to everyone (thought not sure about Spanish citizens). There were organizations in Denmark that very much had an issue with these new requirements. Now, knowing how "fast" Spain rolls things out, while this officially went live beginning of October, it's possible there's an initial grace period. So it's possible they won't ask for your December trip.

This article is written for British citizens, so I know that's not the EU, but it might provide some added info to help out:

https://www.dailymail.co.uk/travel/article-13924089/spain-holiday-warning-bank-details-hotels.html