I think he refers to the fact that WhatsApp sends all data via the phone. The private keys are only stored on the phone. With WhatsApp you have a session key v

I think he refers to the fact that WhatsApp sends all data via the phone. The private keys are only stored on the phone. With WhatsApp you have a session key via the At code you scan. All messages first go to the phone, which decrypts and reencrypts using the session key. Quite sophisticated, but sometimes slow and not working if the phone has no network connectivity or power.