I (perhaps over-optimistically) read it as Brian Sibley is taking the various edited work Christopher Tolkien did on the Second Age texts and bringing them toge

I (perhaps over-optimistically) read it as Brian Sibley is taking the various edited work Christopher Tolkien did on the Second Age texts and bringing them together in a single place, in order, with some sort of commentary or framework around them. Doesn't sound like he's actually re-working the texts themselves, but who knows.