It's hard to know, because the existence of the scene was more or less leaked in this audition tape from last August, where they mention spoilers:end of spoilers That must happen early in the movie, but what happens after that we don't know. spoilers:
that they fought a giant demon that was trying to abduct America. Strange and Wong are almost certainly the people she's talking to in the scene, and the demon must be Gargantos like the box says. end of spoilers
It could be that Wanda summoned the demon and the leakers mistook it for Shuma-Gorath... who knows, they clearly don't know everything that happens.
So I guess all we can do is wait to find out the plot and try to accept that we'll never know for sure that the leakers are wrong. I hope they are.