Same for me.It just doesn't matter how they knew because that whole sequence was odd, stilted and there was probably a better way to write that.
It just didn't work for me.
I mean, there are ways to logically work out how the characters would know. But the whole Woo going, "well actually, some people think Captain Marvel could have bla bla bla..." really just came off as the show acknowledging fan debate, because it's one thing to know the details of the Endgame battle, but they were basically arguing power levels.