To be honest I think it most likely said that because she abandoned her original world, and it considers the fact of "if she's willing to turn her back on her entire realm and what she's known her entire life why wouldn't she do the same thing to you?" Type of feeling rather than an omniscient knowledge that she'll betray MC, besides she's taught MC a lot so far and I doubt she'd risk being trapped in that "void" again or losing all her points and therefore her life by betraying MC, he is her only gateway to more points and lifespan at the moment. Maybe if she wasn't so strictly attached to his existence than she could be a higher risk to betray him but she even swore her knight loyalty, I'm sure there's a hefty penalty for her to pay if she ever betrayed her Lord.
remember in the description for hiring mercenarys it said don't trust her too much i don't think i didn't put that on purpose so maybe she will betray him or some the info she told him were wrong ┑(￣Д ￣)┍ im not sure but that don't trust her too don't trust her too much is bothering me