@PsychoJing I think it was a joke that Tarou couldn’t recognize “him” as a girl even with the swimsuit. That’s why Eve pulled out the passport and insurance card because it officially states her gender.
When we think about it, the last page is so... stupid. It would ruin the whole story, like "Oh, you dropped your ID card... Wait, it says you're a girl on it. Are you really one ?" "Yes" End of the manga.