You're joking, right? You're gonna count all 9 valkyries as being bosses within a "30 hr game"? If they doubled the number of valkyries, would you then have said there's 24 bosses? Come on, that's disingenuous. I didn't run across a SINGLE valkryie in my first play through, until after I "beat" the game. For me, a boss is integrated into the main story, not a side-quest that you can optionally do later on and that has no effect or connection with the linear game.
There are 15 Lynel locations in BoTW, do we add them to the boss count? They're extremely challenging unless one is highly upgraded. At most, valkryies can be counted as 1 "boss".
I killed every valkyrie during the story progression, from the point you get the means to open the ice doors, whose importance was hinted in the very first area of the game.
They are all optional, but not hidden and to miss them is either a deliberate option or a clear lack of interest on exploring anything outside the main objective pointer on the compass.
Also, they are all similar because they get progressively more challenging and introduce new moves in preparation for the final encounter with the queen, a boss that has no reason to be shy comparing itself with the best soulsbourne bosses.
---
Also some people are really missinterpreting the one year per boss comment and comparing with other games, because he is not obviously talking about the sons of thor, trolls, valkyries or dark elf there, but the crazy setpieces with Baldur and I guess the Dragon too.
