I don't see why she needed to be from a prophesy in order for 'nobles oblige' to copy a skill from her. The maid he first had sex with wasn't from a prophesy since he didn't have the prophesy skill yet. And the skill 'equivalent exchange' he got from the traveling entertainter', and she wasn't explicetly mentioned as a prophesy either.
This just seems like the auther nerfing the mc a bit.