I've never heard of Fluid before, but it seems to be a glorified shortcut with minor features such as notification badges. I really can't tell how this can integrate with Electron at all.

Electron would have you write a web app and package it as an app installed to your Applications folder just like any native app. You'll write this code in JS, HTML, CSS and use Electron APIs.

Fluid also would have you write this in JS, HTML, CSS but use Fluid APIs instead. The two compete for the same role in that sense.