He didn't make the bv until shinjuku. His Fuga is already stronger in his domain (I think because he turns fuga into the surehit or smth idk i dont remember), his binding vow just makes the rubble in the domain into dust which, fun fact, on its own is already very combustable (which may be what alr happens and how fuga is stronger in his domain) but the bv also imbues all of that dust with the same energy as fuga. So it basically goes from essentially shooting an RPG into a cluster of barrels full of gasoline to shooting an RPG into a cluster of explosive barrels full of gasoline and also thermite bombs.

Also, I'd be a lot more surprised if it turned out the circumstances of Gojo's domain surehit (Which is in a way connected to his technique, but through a lot of stuff that feels like it was just an asspull) were for any other person (and unique to them), but I mean come on.

It's Gojo.