From the trailer, it looks like the film literally picks up from the last scene. At least Bob and Helen fight the Underminer, kids shown dressed up but not fighting. It also showed Bob getting bested, if not briefly, by Underminer.
I figure Helen got the finishing blow and she saved the day. People saw it and it was big news. Guy who wants to bring supers back wants to start with her because of her being a hot commodity at the time. Or because they'll probably explain in the world and or time period people are tired of strong guys or just guys and want a woman so she is relatable or marketable.
Only way I could see this working. I'm hopefully optimistic, but Disney AND Pixar have fucked up sequels more than once. I just hope they don't ruin the franchise.