Probably something to do with phoning home to Facebook.
We may collect, use, preserve, and share your information if we have a good-faith belief that it is reasonably necessary to: (a) respond pursuant to applicable law or regulations, to legal process, or to government requests; (b) enforce our Terms and any other applicable terms and policies, including for investigations of potential violations; (c) detect, investigate, prevent, and address fraud and other illegal activity, security, or technical issues; or (d) protect the rights, property, and safety of our users, WhatsApp, the Facebook family of companies, or others.
Third Parties. WhatsApp may transfer data within the Facebook family of companies and to third parties, including service providers and other partners. In accordance with the Privacy Shield Principles, WhatsApp is liable for any processing of personal data by such third parties that is inconsistent with the Privacy Shield Principles unless WhatsApp was not responsible for the event giving rise to any alleged damage.
Service Providers. In conformance with our relationship with PSPs, WhatsApp works with service providers including Facebook. To send payment instructions to PSPs; maintain your transaction history; provide customer support; improve, understand, customize, support, and market Payments; and keep our Services safe and secure, including to detect, prevent, or otherwise address fraud, safety, security, abuse, or other misconduct, we share information we collect under this Payments Privacy Policy with service providers including Facebook. Facebook will have no access to encrypted BHIM UPI transaction information in clear format. When we share information with service providers, we require them to use your information on our behalf in accordance with our instructions and terms.
Since WhatsApp isn't open-source, you can pretty much assume they're lying any time the policy says they won't share something with Facebook.