As it says in the description, the depicted characters are either alicorns or of equal power as an alicorn. Trixie had the alicorn amulet, which (arguably) made her as powerful as an alicorn.
And Chrysalis was able to beat Celestia, so she's definitely as powerful as one (provided she gets to feed of a strong source of love).
Besides, we all know that the Great and Powerful Trixie will become an alicorn eventually xD