The story's focus is certainly on Mathilde, but the narrator does not speak from her point of view. Instead, he talks about Mathilde as if he were from the outside looking in. When he brings her up at the beginning, she's just "one of those girls" (1). It sounds like he's seen a lot more of them than just this one.
That's omniscient, all right. Mathilde's also not the only character whose thoughts he can see into; he's able to speak into her husband's thoughts just as easily, when he wants to.