Yes, that was explicitly stated in an interview[1] a while back. Quoting from the specific section:
> “Okay, well, WhatsApp — we have this very strong commitment to encryption. So if we’re going to interop, then we’re either going to make the others encrypted, or we’re going to have to decrypt WhatsApp.” And it’s like, “Alright, we’re not going to decrypt WhatsApp, so we’re going to go down the path of encrypting everything else,” which we’re making good progress on. But that basically has just meant completely rewriting Messenger and Instagram direct from scratch.
