That doesn't mean Big Mom was a threat to Shanks. He was waiting for Kaido's demise, but as luck would have it, he took Big Mom with him.
What headcanon? Nothing happened to GB's hype so far. Again, it's not my fault if you're expecting him to be as durable as Kaido or Big Mom. Shanks lost his arm to a sea king, remember? He's still far stronger than Big Mom.