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

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.