Really a great chapter I love it.
I am leaning toward Shanks talking about Blackbeard instead of Luffy to the Gorosei.
About big Mom and Kaido past. I do think (just a theory not fact) that Katakuri is the son of Kaido and Big mom. Which explains why he is the strongest commander of Big Mom 's pirates.
- - - Updated - - -
Well tbh this is not the first time Shanks did it. He did it during the MarineFord suddenly appeared out of no where without anyone noticed if you remember.
I do think there is a theory that Shanks devil fruit might stop time or his haki is too strong to the point it stops time temporary.