The actual law is cited, so perhaps law changed since you learned it. Or you are just incorrect. According to German law and court decisions, it seems quite definitive. Although I know very little about German law, perhaps there’s some local law that supersedes regional, national, and EU law.
What I try to do is track to the actual source rather than relying on my own memory of things.
I see no link for the [2] in your comment. I sincerely doubt I'm wrong about this, you can read about it for example here: https://hoesmann.eu/fotos-von-gebauden-personen-und-marken-i.... It also explains under which exceptions pictures of a single person can be valid anyway, without explicit permission.
> According to German law and court decisions, it seems quite definitive.
That's impossible. Basically nothing is definitive in the german Medienrecht ;) Ok, not really true, but it's true that things can change and that it is a less defined area and you'd have to be really certain to know the relevant case law to be almost certain here.
> Nach dem Bundesverfassungsgericht (BverfGE NJW 2000, 1021) ist bereits ab diesem Punkt [that a photo is taken] ein Kontrollverlust der ohne Einwilligung abgebildeten Person über das Bild gegeben, und dieser mögliche Kontrollverlust rechtfertigt sogar unter Umständen ein Fotografierverbot.
But that is debatable and actually the point that could be outdated.
In practice, the guide is not completely wrong: You can take pictures in public spaces and they can show people, but those people should not be the focus of the image and you might want to make them unidentifiable if you publish the image.
What I try to do is track to the actual source rather than relying on my own memory of things.