Right now, the Pixelbook is the only Chrome OS device with Google Assistant. Google said the feature would come to other models at some point, but progress seems to be slow. More evidence appeared in August that proved Assistant was still being worked on, and now it has finally gone live in the Canary channel.
If your Chromebook is on the Chrome OS Canary channel, you can probably enable Assistant right now. Just copy and paste chrome://flags/#enable-native-google-assistant into your address bar (clicking the link won't work), set it to Enabled, and reboot your device if asked. The flag should appear in the Dev channel soon.
Google Assistant for the Pixelbook is just the Android app in a slightly different design; it's not clear if this is a new native implementation, or if Assistant will be limited to Chromebooks with the Play Store. Google says "all Chromebooks" in the flag name, so hopefully it's the former.
- Chrome Story