On my windows pc chrome only reacts to ok google when you have a new, empty tab, or the google search page open and active as the main tab. Additionally i checked with procmon what network activity chrome was making, and while it starts sending stuff AFTER "ok google" is activated, it doesn't send any between me saying it and chrome confirming it.
The theory that it's a small local plugin is also affirmed by the fact that my cellphone can do "ok google" without any sort of network, and is sometimes tricked into activating by audiobooks that make noises completely unlike "ok google".
I found the plugin installed, but deactivated by default. So, for the story of this dude to make sense, he'd have to have someone at his computer activate it (or if he's using a linux distro, the maintainers might've activated it by default), and would see the light blinking on new tabs and google search pages.