I think you’re the only person here so far who read the linked article.

I also found the netzpolitik article that was referenced but not linked to in the original article.

This describes police adding a linked device to a person’s WhatsApp account while gathering evidence (translated with Google Translate).

On January 12, 2020, Mr. and Mrs. P. were questioned. During the questioning, they voluntarily handed over the mobile phones they were carrying to the interviewing officers for a brief period so that messages from their daughter contained on the devices could be viewed and, among other things, photographed.

While the photographs were being taken, the computer-based application WhatsApp Web was covertly activated via a website made available online by the Federal Criminal Police Office (BKA), allowing the messages to be read on a BKA computer (sic!). This did not involve any intrusion via a Trojan horse or similar software.

The only link to Signal here is the nation state campaign which used social engineering via a phishing message from ‘Signal Support’.

This nation state campaign was originally reported by a researcher at Citizen Lab.