It's using Svelte, I wouldn't exactly call that unknown. Why maintain your own library when a third party one does exactly what you need?

Unsurprisingly there are many frameworks/initiatives that end up falling by the wayside over the years, e.g. MacRuby was being lined up to supersede Objective-C for app development at one point.

Didn’t SproutCore become Ember [1]? Just my vague recollection, not sure if that was the case though. Anyway, Ember is still used and maintained, despite not being very well known.

1. https://emberjs.com/

I don't know if it's still true, but Apple Music was indeed Ember at a point.