Context for the False Alicorn status is that Twilight's handling of all the Alicorn magic failed and had inadvertedly split off the magic of Luna & Cadance into her partner. Portal Bump is barely able to handle the physical strain of it all as a mere pegasus. She's on the physical brink as it is, all it needs is one little Tirek-stealing-magic shock to tip her over the edge.
This whole moment in the AU is meant to be the signifier of things changing from fairly standard "Fun Cute Selfship Moments With Occasional Plot!" to "Portal Bump is as much of a part of Equestria as the Mane 6, the Princesses and all their friends, and she needs to adapt, or else."