[zurn]

> IMEI is intended to be unique and private

It's intended to be unique but not secret and not hard to guess. It's a bit like your SSN or a computer's MAC address.

> If someone knows your IMEI they most likely have enough control over the phone to either completely spoof it or put malicious software on it

Err, no? Your phone can be asked to broadcast it via radio, your phones previous owner / sales clerk knows it, etc, your wife/gf knows it, etc. Now it's trivial for any of those to gain access to your WhatsApp without any active and sophisticated attack requiring physical access.

Sure, with sufficient effort it might be possible for someone sniffing radio or having at some point handled your phone to subvert it in other ways, but this is zero effort.