Tldr: This is a traffic analysis attack, it exposes metadata without help from or accessing whatsapp data. Other messengers are vulnerable too. It requires vast resources and access only governments have.

The interesting part of the article ist the last one.

According to the internal assessment, the stakes are high: “Inspection and analysis of network traffic is completely invisible to us, yet it reveals the connections between our users: who is in a group together, who is messaging who, and (hardest to hide) who is calling who.”

The analysis notes that a government can easily tell when a person is using WhatsApp, in part because the data must pass through Meta’s readily identifiable corporate servers. A government agency can then unmask specific WhatsApp users by tracing their IP address, a unique number assigned to every connected device, to their internet or cellular service provider account.

WhatsApp’s internal security team has identified several examples of how clever observation of encrypted data can thwart the app’s privacy protections, a technique known as a correlation attack, according to this assessment. In one, a WhatsApp user sends a message to a group, resulting in a burst of data of the exact same size being transmitted to the device of everyone in that group. Another correlation attack involves measuring the time delay between when WhatsApp messages are sent and received between two parties — enough data, the company believes, “to infer the distance to and possibly the location of each recipient.”

Today’s messenger services weren’t designed to hide this metadata from an adversary who can see all sides of the connection,” Green, the cryptography professor, told The Intercept.