Exactly this. Basically what the GDPR says is: if your business doesn't require the data, you cant use it without the user's consent. And data used for better advertising is NOT essential to e.g. a news site.
What's more, the regulation syas that you can NOT simply say "accept or leave" in that case. You then have to provide the service to the user without storing that non essential data. You can't provide a service, even for free, that you condition on storing data not essential for that service. There is no "if you don't like it, leave" clause.
Basically: spiegel.de has to be prepared to show their news to anyone, including those that do not wish to be tracked by their ads. Right now we are in a period of denial where site owners believe they can have these "By entering you agree to..." banners. Once the first large fines are handed out, It'll be fun to watch.