I assume that this app is not a mailing app, its an app to control your fastmail service, which includes email, file storage, webhosting, various dns controls, notes, contacts, billing etc. They already contributed JMAP. It would be impossible for them to get all of those features into k-mail - k-mail wouldn't want them.
And packaging the app in electron is relatively easy and a nice convenience for some users.
The Electron thing is a waste of resources for a bad application.