One of the highlighting features of the Google Pixelbook at launch was the native integration of Assistant in the OS itself. It can be activated by either using the "Ok, Google" hotword like normal or via the dedicated Assistant key near the spacebar. But according to XDA, who have been playing around in the Chromium Gerrit, Assistant might be coming to other Chromebooks in the near future.
The merged commit that XDA spotted mentions "add Assistant feature flags changes accordingly" and "Assistant service provides core Google Assistant functionality." Both of these imply that Assistant integration in Chrome OS cannot be too far off.
Continuing onward, it seems that OEMs will not have to do too much work to enable Assistant, either. They will simply need to enable the feature that tells the OS to listen for the activation hotword, or they can set up Assistant to launch on a specific key press.
Honestly, I expected this to happen. Assistant is a flagship Google product now, thanks to it being an integral part in many of the company's software and hardware ventures. Who knows when we will see it hit the third-party Chromebooks, but I very much look forward to it.