She doesn't even need to be dominant to reject his blackmail. Even if she's normally very submissive. Him threatening her children, is enough to make her very angry and behave in a dominant way.
I think the choice where she backs down and surrenders to him, is just very uncharacteristic for her. Even if she's very submissive and might back down if someone threatened her. When it comes to her kids, she always behaves in a very different way. So for me, that choice feels totally wrong, whatever version of Myriam you're playing. And it's noticeable that's the only scene in the game ( at least where she's been given a choice), where everything about it is negative from Myriam's perspective. Even with Anthony's worst scenes, she consoles herself that she's doing her duty as a good partner.