Can they, though? There are spells that prevent teleportation (Dimensional Lock, Dimensional Anchor, Forbiddance, etc.), and it's not inconceivable that Sereni could have had a version rigged that included an exception for her portal. Plus, we've already heard that the stonework here is multidimensional or whatever (and that's why Xykon can't phase through it), so there's always a chance it naturally foils teleportation or planar travel as well.