Well, it wasn't necessarily a concession. But that's certainly a valid way to read it.
And even if it wasn't a concession, it was certainly a defeat. She got beaten.
She ran away from the fight, conceding it because she couldn't fight against Mantellum and win.
Plus she can't take out Scion or the Endbringers. (I'd mostly let her use her normal stats against them, incidentally. Even if she can't use her power against them directly she can set goals like hitting the space that Leviathan's head happens to be in. If you don't like that interpretation, you may want to add a Limitation to Contessa's Powers.)
I don't think Limitation would cover it, since this is essentially an offensive blocking action by another character.
And since she didn't drag Siberian back to Cauldron, it can be inferred that she wasn't able to. She had every reason to do so, if she was capable.
Given that she was basically the one running the show, since Cauldron was following her plan, I think I'd read that as her allowing Manton to escape rather than her being unable to subdue/kill him. After all, her power isn't something as concrete as projections; it's an I Win button that applies to all but a few beings in the multiverse.
Um, no?
A power like Mantellum's should affect her skills as well as her Powers. Unfortunately that'd be a bugger to stat up, but that's normal, since Worm isn't a DFRPG game.
Fantastic Fists/Athletics is Usain Bolt/Muhammed Ali level of skill, which isn't how Contessa wins. The one occasion we see her go up against someone she can't use her power against, she runs. That says to me that when she's bereft of her power she's just as human as the rest of us. Going further than that, Epic Conviction and Scholarship should really only apply when she has her power.
Personally, I'd just have her power give a static bonus instead of putting her base stats so high (+2 to mental skills, +2 to social, +1 to physical. If I was going to stat her, that is). It's powerful, yes, but Contessa's power is so OP she could kill the single most dangerous cape in the multiverse without there being any threat to her.