Gilmoy: It might be a KIA by dint of <not quite> being anything else, but it sure doesn't play like one. <5.d4> delayed Sicilian seems to favor Black's Bg7, and <6..Qb6 7..Nc6> unbooks whatever prep White had -- now he must fear that he's falling into Black's prep, if Black was so eager to force this issue.
<8.Nc2> feels meek: not just the OTB result of retreating a slow N (already White is shedding his Sicilian space advantage), but the off-board meta-game hint of ducking a prep mud wrestle.
<8..Qa6> is only an irritant, but Black is gradually steering the boat. <11.Kxe2> seems safe (which is why White offered it) -- but Black energetically turns this tiny detail into <the> key strategic feature on the board, with <14..Ba6> way stronger than the Bg2 still hibernating in winter.
<15.b3> is an odd way to "devalue" the Bg7: just so he doesn't have to defend b2? Meanwhile, Black's Rb8-b5 has done the same to Bg2. Tactics from <16..Bxc3> morph the board into a b+c vs. c+d rook duel.
With both Ks centralized, what principle remains? "Knight Pawns Lose", perhaps: the naked minority attacks <25..a5 36..a4> and <32..h5 33..h4> open too many highways, undressing White's <29.Rb2> doubling: his vertically-split Rs can't challenge any open files!!
Maybe it's the inherent weakness of fianchetto pawn structures after the Bs are gone (which already suggests a midgame plan of trading them). Black had no knight pawn "weak" points because he traded his off :) A possible pattern signature, to file away for future matching: <Fianchetto No Rook>. With more exemplars, possibly to become a (gasp) Law ...