> maintaining all your documents in one place, annotating them with highlights, comments, managing your documents with tags, and maintaining key extracted knowledge via spaced repetition

> I believe this is a new class of application

I haven't used SuperMemo (the app that inspired Anki development), but looks like it has all these features plus tons of other stuff (https://help.supermemo.org/wiki/Incremental_learning). It has some flaws (not cross platform, doesn't support PDF directly, complex UI, and of course closed source), but I think it's unfair to claim "a new class of application".