Well of course, you can receive notifications from WhatsApp and send replies. That works on Apple Watch too, including over the cellular network, not just Bluetooth.
But that functionality is extremely limited. You're unable to do things like initiate a new message to a random contact, or browse through a list of messages or group chats. I can't imagine how this stuff can work on Garmin if there isn't a native app.
Your second screenshot is from a phone connected to a watching running Wear OS (Samsung, Google, etc watches). Wear OS *does* have a native WhatsApp app. But Garmin watches do not run Wear OS.