It's not surprising where you see a lot of images by artists where a character doesn't have a particular feature (e.g. Aya's wings, Cirno's wings, Ran's tail etc).
If you want to get into semantics - she IS an Ex boss. After all, phantasm is a "harder" Extra stage, thus I see nothing wrong with her being listed as an Ex boss, and this image being listed in the pool is fine.