I notice that in the text, the are often titles which appear in square brackets. For instance, on page 153, there is reference to an [Allegro] in G minor and a [Polka] in C minor.

Are these bracketed titles some sort of "inferred title" that Sibelius did not actually provide?

Yes, exactly. Many of the early pieces and fragments were left untitled by the composer - even quite large movements. Most of these works have been provided with ‘working titles’ by the musicians who have performed and recorded them - not least Folke Gräsbeck - and such ‘unofficial’ titles are indicated in square brackets. They are based on the music’s style and character, and correspond to the types of title that Sibelius himself allocated to similar pieces. As they are an unobtrusive and convenient way to identify a significant body of works, I chose to retain them. One cannot pretend that they are authentic, but if we just said '24 bars in A minor' (or whatever) it would get pretty unfathomable!