Yeah “stock” iOS is the version from Apple. I was trying to convey the additional optional security features that put it above all versions of Android, except Graphene. 

WhatsApp is a third party app, created in china, purchased by Facebook. The real answer is don’t use it. But the danger of using WhatsApp is the same between Android and iOS. Again I’m trying to point out Graphene stands on its own here.

Cybersec researchers work pretty hard to breach iOS and their work is in the open, even if the public can’t look at the iOS source code.