Where does it say that? The linked article says "recordings of their house", which very well could e.g. be a camera on the outside, capturing surrounding public space.
(also probably existing law, not GDPR specifically: video surveillance has been fairly strictly regulated for a while)
It's not the GDPR that made this illegal. It was most probably illegal before the GDPR, and it was probably enforced by the same agency that now enforces GDPR. The GDPR is an umbrella that covers all the new things it introduced, but also a lot of old things the various national data privacy agencies covered.
Recording in one's own home is exempted under the GDPR[0].
I suspect something broader was involved here.
[0] Article 2(2): "This Regulation does not apply to the processing of personal data [...] by a natural person in the course of a purely personal or household activity"
A prime example where GDPR would apply to a security camera in your own house would be if that camera was used to record renters (including short term rentals e.g. AirBnB) without their knowledge.
For example, I recall reading about cases of renters finding out that the landlord has installed hidden cameras in the bedrooms and showers.
(also probably existing law, not GDPR specifically: video surveillance has been fairly strictly regulated for a while)