While Assistant is better integrated with Google's own services, its support for third-party messaging apps has been commendable. It can send messages via WhatsApp, Telegram, Viber, Facebook Messenger, and others, record and send voice notes from WhatsApp, as well as read incoming messages from many apps. However, it didn't support calls on any of these apps, but it's now adding that for WhatsApp.
Until now, voice calls initiated from Assistant on your phone simply placed a network call to the person you wanted to contact, while video calls could go through Hangouts or Duo. Assistant is adding the ability to use WhatsApp instead for both, which is very, very welcome in countries like mine, where the messaging service is the default option for calling anyone due to the exorbitant network call fees.
You should be able to say "Hey Google, WhatsApp video John" to get Assistant to make the call through WhatsApp. Google says the command is available "now," but it isn't working for me yet. It could be slowly rolling out. We'll let you know once it does and we'll try to find what the correct command is for voice calls through WhatsApp.
The announcement doesn't specifically clarify if this feature is for phones only, but I'm willing to bet it is. Just like messaging, it simply passes the command to the WhatsApp app, and that's something speakers and smart displays don't have.