The ability of stealing someone's Celestial Spirit was something she had. She could steal someone's Celestial Spirit with the Book that she always carried, and that's why she knew just how strong this ability was.
Even when she used the book and stole someone's Celestial Spirit, she couldn't erase their soul mark. She could only erase the Stars in their Celestial Spirit Marks. At best their marks were left a little faded, but in your case, the entire mark had disappeared.
It was as if someone devoured the entire Celestial Spirit which even she couldn't do. Some unknown person possessed an ability, similar to what the mysterious book granted her! And that person was watching the City at the moment, remaining completely under her radar.
For a moment, she thought if it was another heaven's chosen.
