That page is referring to the native Linux binaries that Unity can export (which work great and has been available for a while). It's also quite stable so not sure why they're still calling it a preview.
The parent post is referring to the Unity web plugin for Linux which AFAIK does not exist at all (only Windows and Mac support for the browser plugin).
googles "Unity Web Player" linux ... cries
P.S: I'm on Chrome and UWP is supposed to work on it, but it doesn't.