GB never said he wants to take on the whole alliance. He knows they're weakened, and he also says his only target is Luffy. I mean, he'd obviously meet with Luffy's allies if he's gonna attack him, but that doesn't mean he was stupid enough to think he can take on the whole alliance.
Also, if Shanks let out his CoC after sensing GB, it speaks volumes not only about his CoC but also his CoO—Especially considering the fact that Shanks' crew hasn't even climbed the waterfall.
If GB sensed Shanks without Shanks doing anything, then GB's CoO is crazy.
Yea, it is such lazy writing. He does this all the time and I don't know why other than him just being lazy and not wanting to do his homework. It feels like he is too busy with other nonrelated manga work that cuts into his time, to actual plan and develop these characters right.