I believe it was indeed a web app. The efforts to get a shared code base began in earnest after Nadella took over as CEO at MS, and Office stopped being a means to sell copies of Windows and rather a chargeable service unto itself - I believe the shared codebase first started to surface in the apps for iOS and Android.

Well, I say that. Really, the Windows and macOS version had been sharing code for ages, but plenty of it was still bespoke for macOS. I think their rewrite to Cocoa, which I believe happened around the same time as the iOS apps started being developed, helped to bring further parity.

> According to this post [0], Microsoft use a "shared Office codebase for Windows, Mac, iOS, and Android".

At very least, from my experience, I believe the iOS, Mac and Windows do share the same underlying engine. The web version looks like a reimplementation that reimplements all its weird quirks… but there're still minor difference here and there.

I have to deal with few Japanese documentation which uses a relatively obscure feature exposed only to Japanese version of Office (e.g. I think it present in all versions now as "Enclosed Character") and so far every Word implementation fail to render that in the way it's expected to be rendered, but it renders consistently in all versions of Microsoft Office {Windows, iOS, Mac}.