The review on that page leaves much to be desired...
"Until now, classical music listeners have not had an adequate context in which to place Sibelius's well-known and much-performed works..." - James Hepokoski, Yale University
While I'm sure the book is fine, what makes the reviewer think that listeners do not understand the context of Sibelius' works? I would dare say that the context in which Sibelius' works are written is better understood than with moth composers.