Sure it is, in the sense that it’s very likely not using any Chrome-specific APIs (Chrome right now has the least exposed extension API surface to regular, non-ChromiumOS extensions) but rather at most being equivalent to an Electron app, where some of the HTML and JS lives and runs on the client rather than on a server. But that HTML and JS still is HTML and JS built for a web-rendering-engine to parse and run, so it should be fully possible to turn this extension into a regular web-app with not very much work. Probably any sufficiently-motivated programmer could do it in a couple of hours, throwing the result in an S3 bucket for anyone to access. (Not sure how well it’d run on other browsers, but we still call regular web-apps where the developers happened to only test it in one specific browser “web interfaces.”)
It's not just the web versions of Office that work on Linux. On Chromebooks, the Android runtime ensures that the Android MS Office apps run full-screen and take advantage of the large screen and keyboard[1,2].
