@Qelix: Yup, retcon it is. But it's forgivable. After all it was over 120 chapters ago, way back in chapter 1, that the mangaka asserted that Shirogane was "A man of unparallelled wisdom and intellect" and "A genius among geniuses, the top scorer in the entire country!" :
As you can see, he even scored higher than Shinomiya, so this new back story is a bunch of horseshit.
It's unfortunate that the mangaka has decided to retcon the story and make Shirogane a dullard just so the useless Shinomiya can shine like the light that comes out of Bono's ass.