It is speculation, but only to some extent; upon the acquisition they've pledged to preserve the privacy and security of WhatsApp users.

To some extent the announcement of him leaving, following changes to terms of service last year, make if for a very good canary in the coal mine kind of warning, that even if bound by confidentiality or other agreements preventing disclosure, he's leaving $4B behind for "some reason". I'm sure if it was to launch a new venture that'd be fine to disclose...