You would be totally correct if this was a standing situation. However since the defender is pinned on the floor, the real challenge is not a matter of beating the the attacker's thumb vs beating their four fingers. The challenge is beating the structure which allows the attacker to rest his weight through his straight arms onto the defender's wrists to pin her.
Your proposed alternative of moving her arms inward runs into two major obstacles. Firstly moving her arms inward puts them in a position where the attacker actually has an even better structure for transmitting his weight downward into them. Secondly, unless she has remarkable shoulder flexibility the inward path will actually involve attempting to lift her hands off the ground at some point, which means she would need the strength to lift the attacker's weight using just her arms (and at an awkward angle to boot).
The defense shown in the video clip is not focused on breaking the attackers grip. It's focused on moving his hands outwards and towards his feet so that he can no longer rest his weight on them. As shown, this usually leads to him letting go of her wrists so he can catch himself and not face plant. For her follow up she will most likely use one arm to hug his body close so he can't posture up and strike her, use the other arm to overhook one of his arms, capture his ankle with her own foot on the same side as the overhooked arm, then bridge and roll to reverse the attacker and end up on top. (There are other possible follow ups, but that is the one this particular instructor teaches as the first go-to option.)