I though that fennec was a monolithic implementation for android of the Gecko engine, while fenix has the ui part and the geckoview part separated (so you could create a different app with the same rendering engine, like webview of android).

I have read that geckoview takes the old gecko implementation, probably copied and adapted, but other than that it's a new project with a different development path. That's why I say it's a different browser.

I may be wrong though,I tried to search for sources but unfortunately couldn't find any.