In this case, "all but" is not being used as a contrast phrase. It means "very nearly" (see this post conjunctions - "All but" idiom has two meanings? - English Language & Usage Stack Exchange for an explanation of the idioms.)
You could say something like: "I all but died when I saw my GRE score after the first attempt." Which would translate to: "I very nearly died when I saw my GRE score after the first attempt."
Similarly, in this sentence the sequel is very nearly full of _______. The blank that works here is "bombast" - which is essentially empty language. So in total the sentence is saying: "While her first novel was exciting, it seems that she ran out of material; the sequel was essentially just empty language designed to impress."
It is a confusing idiom, and frankly a poorly phrased sentence. But hopefully the double meaning clears this up!