Yeah, the article is super-vague about that:

> In 2016, WhatsApp added end-to-end encryption, a security feature that scrambles people’s messages so that outsiders, including WhatsApp’s owners, can’t read them. Facebook executives wanted to make it easier for businesses to use its tools, and WhatsApp executives believed that doing so would require some weakening of its encryption.

Not sure what this means. "Weaken" is kind of a confusing term, FB either has access to the message content to feed in to adsense's gaping maw or they don't.