This disappointment might have been lessened if the novel had been called something else, but it also might not have sold as many copies.
I go so far as to say your disappointment would probably have evaporated completely if you'd read the blurb for the book.
That is: this is not like Prospero Burns
except for the title. The blurb and even the concept of the book are massively different from PB. Not only has this not been marketed as a counterpart to another book, it's not got a misleading blurb and also is about something we (if we've been paying attention) already know to be a pretty odd subject matter, not just 'another' battle (even if a significant one).
To that end, I don't think complaining about the title holds any weight whatsoever.
Having said that, I do agree with you: it's a resounding & stand-out success for the series that sets a very high bar indeed.
I'm slightly surprised you didn't mention one of the other 'big characters' of the novel, even if said character only appears for a tiny fraction of the time that Kersh does...